Christmas time is the only time of the year I can take a 2 week vacation....

I would do Italy for Christmas (and would pick Florence or Rome over Naples, but to each his own on that) We have been in Florence that time of year when it was balmy and pleasant and in the low 60sF; it can of course be worse. We were in London for Christmas one year and the weather while cold was not rainy for the most part. This time of year, luck is involved. Have layers (I always travel with silk long underwear in the cooler months since it immediately warms up the outfit) and a good raincoat with lining, waterproof shoes, hat and gloves and then just plan to enjoy what comes.

I woud do Italy for Christmas because London pretty much shuts down on the 25th and 26th. When we were there even the trains were not running on the 25th and local transport was closed down on the 26th. I'd do London for New Years and Italy for Christmas for that reason. London is a town filled with great urban indoor venues and the museums are mostly free. It is a great winter destination since there is always something to do indoors if it isn't a day you want to be outdoors.

Yes, fewer daylight hours in December than August, but your are likely to get wetter (in terms of quantity of precipitation) in August (54 mm) than December (48 mm in 10 days).

http://www.metoffice.gov.uk/public/weather/climate/gcpvj0v07
http://www.yr.no/place/United_Kingdom/England/London/statistics.html

Naples has what we West Coast people call a Mediterranean climate - dry summers, wet winters. Naples climate statistics: August = 4 days of precip totaling less than 30 mm) with December = 14 days of rain totaling 100 mm.
http://www.holiday-weather.com/naples/averages/

As the Norwegians say, no such thing as bad weather, just inappropriate clothing.

We spent last Christmas in London and have spent four Christmases in Rome (2 as travelers and 2 as residents).

My comment on London is that we loved it, but because of the myriad closures the 24th, 25th, and 26th, you should consider Italy for that week, then to London for the 2nd week. Italy does not close down quite so hard as the U.K. for the holidays. Trains run, for example.

Italy in December is not warm, but your chances of rain are not terrible. We have had "shirtsleeves weather" occasionally the end of December (i.e., above 60 degrees Fah.)! London was chilly, but not miserable. Warm coats, scarves, gloves, of course. I think I used my umbrella 2 or 3 times in a full week there. Similar dress for Italy, and peel off the coats if you get that nice over-60 weather!

Why Naples?

My problem with London in December back in 1985 wasn't the temperature (coming from Philadelphia at that time, it didn't seem cold at all). And it wasn't even the short daylight hours, although they are indeed short. It was the fact that it never got very bright out, even at "high noon." I found the lack of light during the day depressing (and again, I was comparing it to Philadelphia winter light; coming from San Diego, you will find it very gloomy). I agree that once darkness hit, I was actually happier; lights go on, everyone is out and about, and I felt a lot less gloomy.

Do take seriously the part about London shutting down completely for Christmas Day, and being only partly open on some adjacent days.

We go to London almost every year in either November or December and we LOVE IT. We've never had a prohibitive amount of rain - never. Some days maybe some drizzle - a steady-ish rain in the morning on another day - and some grayer skies - but also bright sun and bracing temps that are PERFECT for walking!

And London around Christmas time is absolutely MAGICAL. I know I am using a lot of ALL CAPS here, but I can't emphasize enough how fabulous it is!

Off the top of my head:

Your kids will adore going to Covent Garden (jugglers, singers, comics, music, madly extravagant decorations, amazing shopping and food).

If they like a youth-y vibe and trendy shopping, there's the Seven Dials neighborhood - many fun shops and excellent little eateries, with cheery, on-trend Christmas decor.

Somerset House has a luxe, traditional, VERY Christmas-y market set up inside its very Ye Olde 18th century corridors and side rooms, and a skating rink out in the main courtyard!

Oxford Street is all decked out with twinkly lights - every store strives to out-do its neighbor.

Carnaby Street does Christmas decorations rock-n-roll style - very fun to visit, stroll around, shop and eat.

PLEASE don't let yourself be put off by any Debbie Downer talk about "short-daylight-hours-it-will-rain-it-isn't-Florida" talk. London at Christmas is the BOMB. I am already planning this year's trip!

Have fun!

I love winter travel in Europe... I have been to London,the Cotswolds, Germany, Austria, Parisand Italy in winter, and in my opinion I would rather have some cool weather and possible rain than hordes of tourists and sweltering weather.

While it can be a bit grey and cloudy I prefer to think of it as atmospheric instead of gloomy... It is also much better for your photography than the bright sunlight and heavy shadows:)

I also don't mind the shorter days as I love seeing the city lights come on earlier so I can enjoy them. Just makes it all feel cozy to me.
Be sure to dress in layers and I think you will be pleasantly surprised. Of course, you San Diego people are a little spoiled...
If you are able to make any of the Christmas markets they are great fun. I always feel I am much more a part of the culture when there are fewer tourists and I can enjoy my vacation at a more relaxed pace.

We have been to London early in the Christmas season (Nov) a couple of times and loved it. We loved seeing all the Christmas decorations and shop windows dressed for the season. The first two weeks of December 2015 we were in France, Germany and Austria. The weather was fairly mild (40's) and only one day of rain. We were pleasantly surprised. Of course, you can expect rain in London. I wouldn't hesitate to go back to Europe at Christmastime. We would even consider going in January or February. As others said, dress appropriately and you'll be fine.

We traveled to Paris several times in January. One January we did a Segway tour - there were 4 in my group and one other person had emailed about the same date so they opened up for us! Just dress appropriately and have a great time! you will love how un-crowded places are.

I'm curious why Naples? I'd go to Sorrento, which has the best train and bus connections to Pompeii, the Amalfi towns, even Naples not much more than an hour away and there are ferries to Capri.

I like the idea of starting in London, with the pre-holiday atmosphere and decorations. Plan carefully for December 24 (a lot may close very early), December 25 (of course) and also December 26 (Boxing Day) which is a holiday too and much will be closed - possibly even some trains. Then moving on to warmer, drier, sunnier southern Italy.

I would not go to Pompeii on a rainy day (it's hard enough to walk there when it's dry) but I did end up seeing Herculaneum in the rain and it was still a very good day.

I lived in Bavaria for four years and loved it, but hated the long Winters. Perhaps my opinion is colored by the fact that I am a Southerner (USA) and prefer warmer weather.
I would never spend good money on a trip to Europe in the Winter. The days are very short. It is cold. Not the best situation for site seeing. If you want to go somewhere in Europe in the Winter, try a warmer place like Spain. Naples would be better than Northern Europe.

Also, consider travel to the Southern Hemisphere. Australia is fantastic, as well as South America. Take a cruise around the Horn of South America or visit Buenos Aires, or Machu Piccu in Peru.

We spent two weeks in London last December. We enjoy holiday lights so early sunsets were fine with us. Weather was fairly warm with occasional rain. There is so much to see and do. There is a wealth of Christmas info on the internet starting usually in September. Have a great trip!

And if you come to London, make sure to score tickets to a few Pantos. Not just for the kids - a fair few well-concealed saucy jokes are definitely for the parents :) I used to see at least 1 each year, because christmas without a panto is like a roast without yorkshires!

If you have ever been to Seattle in December, that's what you can expect in London in December.

Tom....soft diffused light, soft shadows, overcast skies, drizzly days, atmospheric fog can produce some wonderful images.

I am a professional photographer and you can see some images that were produced in this winter light on my facebook page
Terry Kathryn Lawrence Photography.... don't want to discourage anyone from doing photography in all types of lighting:)

We were in London for New Years a couple of years ago. The weather was cold, rainy and windy and we had a wonderful time in spite of that. We had been to London a few other times, and for this trip focused on indoor venues we had missed on previous trips. We did the Churchill War Rooms, the Victoria/Albert museum and went to Selfridges. (PBS be blamed.)
We went to hear the Messiah at St. Martins and the Nutcracker at Covent Garden on NYE.
I think magical is a good word to describe the experience. However, I would not recommend that time of year if it is your first and/or only visit to London.
Personally I think it would be easier to pack for just one climate. If you want a different venue I might suggest Germany, Austria, Hungary. You will have an equally magical time seeing the Christmas markets in those locations.I might choose to do one of those first, and end up in London for the second week which includes NYE.

Gorgeous photos Terry Kathryn. As an amateur phtotgrapher, I totally agree that rainy, foggy, misty days are great to shoot.

Thanks Mary... It is a lot of fun for people photograph in lighting situations that they are not used to.... the eye does not see a scene the same as film or (digital) does, so there are often some wonderful surprises. Anyone who has photographed in bright sunlight with deep shadows probably has lots of disappointing images... so just because you travel in winter, don't put away your camera.

The author of a blog I follow went to Venice over Christmas and absolutely loved it. Fewer crowds, decent weather. Just my 2 cents.

I too have to travel when school is out. I work for a college.

Thanks @MrsEB! :-)