Prague State Opera, Saturday night...where to eat?
If you prefer to eat nearby there is a restaurant called Garden in Opera. It is located in the tinted glass building from the 70s next to the State Opera (if you are standing with your back ot the opera, go left). The entrance should be from the other side than the opera. http://zahradavopere.cz/index.html
Otherwise, as you are a veggie lover, there is this good vegetarian restaurant in Prague with a very nice atmosphere called Lehka Hlava: http://www.lehkahlava.cz/en_novinky.htm
Definitely eat at Garden in Opera. Very good. We ate there at our usual dinnertime (around 6 or 7 pm), being American. Towards the end of the meal, all these people starting coming in. I think they had been to the opera and were coming in afterwards to eat. If you're in Prague, do as the Praguees do!