Buckingham palace

It depends how close you want to be. If you want a good vantage point, I'm guessing no later than 8 AM, maybe earlier if it's a dry day (fewer people are likely to come out in inclement weather). Any later and you will mostly see just glimpses over the tops of other onlookers' heads. Be prepared to be squeezed in -- it's not recommended for anyone who has claustrophobia or crowd anxiety.

The RS guidebook tells how/when to witness the other end of the guard change at St. James's Palace. This draws far smaller crowds so you have a much better chance of seeing the whole thing, but it still does get crowded with tour groups as well as individual tourists who, like us, read the RS book.

In either location, if you go early and stake out your spot, make sure it is not in an area marked as off limits, because if you do then it's likely you'll later be told to move and the good spots will be filled by then.

My husband and I used advice from Rick Steves London and went straight to St James Palace to watch the inspection of the old guard and watched them start their march (we got there right around 10:30, and we were two of maybe 20 people there). We then separated from them and hustled across the park to the Wellington Barracks to watch the new guard inspection and then marched with them to Buckingham Palace and ducked out before the “main show” at 11:00 because the crowds there are insane. We enjoyed a much lesser known and awesome experience watching both guards begin their march to the palace and didn’t have to deal with the larger crowd or waiting around for hours to have a good view.

Personally, unless you are in London for a couple of weeks, I think there are better ways to spend your time, but each to their own.

Agree each to their own.

Truly mega crowds and there are so many other wonderful things to experience in London.

Go as early as possible and wait.

If you are traveling off season, you shouldn't have any reason to arrive before 10:30. We don't travel in the summer so I have no idea what it is like then. If it is something you want to see, go see it. My husband likes it, I do not. We all have different interests. It was always an easy walk by as we headed to different sites.

If you are going in the summer you should be there by 10:00am to get a place by the fence to be able to see, otherwise it’s really not worth going IMO because you won’t be able to see anything. I would suggest going to Whitehall and watch Changing the Life Guard, it’s at 11:00am, and you don’t have to get there really early.