The Reichstag building and the Dome

I would just check back at the end of this month. the schedule for May is not up yet as I see the schedule April is

My wife and I took a guided tour last may (much recommended over the basic dome visit). I cannot remember how long in advance we booked, maybe around four weeks. I would just wait until the tickets open up, even drop them on email about it if you can find an email address. In Germany they tend to be somewhat responsive to the sort of thing.

I got my tickets online on the official website a week or two in advance for the free tour plus the dome. There’s no point paying for a tour, as the free service is excellent and is very thorough. You end at the dome and you can spend as long as you like there, which you wouldn’t get on an organised tour.

I just got my tickets for an English speaking tour on April 16. I don't think you can do May yet but you can for April. And I did it through the government website.

You can book dates "only" 2 months in advance except blocked dates (maintenance, security issues, ...).

Thanks, everyone for your help. I will take another look on the Bundestag website in a few weeks.
I am looking forward with great interest in visiting the building and the dome.

I don't know if they follow "American" IT practices to discourage commercial collectors of tickets for improper sale on the grey market or not. But you need to check the website regularly until you understand their "loading" of available dates. I had to take a second choice of day or time, I forget which, but I got the English tour I wanted. We just had to be be there earlier than I wanted to. Be sure to take your actual passport and as few bags as possible, for airport-style security.