day tour to downton abbey

If you're talking about Highclere Castle, you can find info about visiting it here:

http://www.highclerecastle.co.uk/

If you're just talking about seeing where outside locations are shot, you can google Downton Abbey and check the wikepedia site, I think there's a list on there.

Highclere is only open to the public on selected spring weekends and certain days of the week during a portion of the summer. The family still uses the house seasonally and the rest of the time they rent it out for weddings, conferences, etc.
I went this summer and bought tickets the moment they went on sale...literally. I set my alarm to go off at midnight the day tickets went on sale (England time) and it took 4 hours to get them because the website was overwhelmed and really slow. By the time I got on the page to buy tickets the first 5 weeks of their summer opening were gone and, luckily, August still had a few left on days that I could go. You really have to think and plan ahead to do this. There are organized tour companies that you can take from London but they too fill up fast and are expensive. When I got there to tour the castle they were turning people away at the ticket booth by the car park. You can't just walk up and expect to get in.
If you go to this page on their website, they post the date that tickets will go on sale for the upcoming year.

http://www.highclerecastle.co.uk/visiting-us/opening-times.html

It looks like all their weekend spring events are already sold out and they will post information about when tickets for summer will go on sale sometime in February. Good luck!

Highclere Castle just opened their summer schedule. Admission tickets are now available online.

I've visited Highclere the last two summers, having purchased tickets for both years months in advance on their website. In 2012 it was very crowded and almost impossible to move through the house. It didn't help that it was pouring rain and everyone was soaking wet. Last summer it was a lot less crowded and even the inevitable rain was much easier to cope with, as it was more of a "heavy mist." At least it didn't stop us from touring the gardens!

I was told last summer by the ticket-seller in the car park that since the DA craze has cooled down somewhat (RIP certain characters!), you could just turn up any day they are open, especially in the afternoon, and have a very good chance of getting in. This advice worked for my coworker last September 4; your experiences may vary. I just wanted to say it's worth a shot even if the website says they're sold out for the day you want.

Also, the three Rick Steves guidebooks for the UK (London, England, and Great Britain) all have a page on the logistics of getting to Highclere Castle, either by driving, taking public transport (train, then taxi or private shuttle), or via a private tour company. So you might want to have a look at one of them.

Before leaving on our trip to England we tried without success to get tics for Downton. But after leaving Kent (Sept 8)and heading to LHR (did not drop car at LGW in order to have it for DA) we took a chance and stopped by. However, it was a Saturday and a wedding was taking place there. A guide at the gate said we could tour it the next day and so we did. It was well worth it! They were not turning anyone away--walkups to tic booth were happily buying tics. The crowd was fairly large but the tour was well organized and we saw more rooms than we expected.