My husband and I will be in NYC from 12/21 to 12/24 this year. I know that you can get some great ticket prices at the TKTS outlet on the day of the play. However, I need to know if anyone thinks tickets to Mamma Mia and Urinetown would be available. I have never been to NYC at Christmas before, and don't know how busy it may be. Should I go ahead and pay full price to be guaranteed the tickets?

Mama Mia, to my knowledge, has not been available at the Tkts booth. Urinetown is currently available at Tkts. There is no way of knowing what the situation will be around Christmas. It is a very busy time in the city.

Traditionally, that is very busy time for the theater in NY. The chances for discount tickets to Mamma Mia are slim to non-existant. I'd recommend getting tickets now (at full price, of course) for that one. It'll most like be sold out that time of the year. No matter what, don't expect to find it at TKTS at that time.
You've a better chance with Urinetown. However, there's no guarantee. And, you can wait a while before getting tickets.
In general, you probably won't have a great choice for Saturday, 12/21. The next three days should be better, relatively speaking....but probably no Mamma Mia.

repeating what has been said--Mamma Mia is basically sold out completely about a month in advance, but for "good" tickets, you should try 3-4 months ahead. Christmas week is very busy for the theater

Also keep in mind that you aren't going to be in the city that long so do you really want to eat up your days waiting in line at TKTS? Pickings will also be slim that time of year. I would recommend getting tickets to Mamma Mia and the Radio City Music Hall Christmas Spectacular. The City will be PACKED!

Thanks! We already have Radio City tickets and will get right to work on the Mamma Mia ones. Thanks again!