Most English friendly performances listed on Theater in Budapest are sold out quickly due to being played only once or a few times. So usually we recommend to buy your tickets as soon as possible. But if you are lucky, you can buy tickets with big discount last minute:
Ma este színház (‘Theater Tonight’) offers last minute tickets for half price+fee on the day of the performance. Sometimes concerts and even guided tours in English are included. You can sign up for their newsletter which you will get right after midnight with all available tickets for the given day.
Unfortunately it is only available in Hungarian.