Very interesting topic! Like many said, it really is somewhat of a moot point. The night of April 16 I decided that I would not drink the following day. A local radio station was playing 24 hours of Grateful Dead music on April 17, and being an old Deadhead also, decided on the 16th not to drink on the 17th and enjoyed a great soundtrack for my first day not picking up a drink!
For me, it is more of "the weekend I quit" than an exact date. I went into it drunk and ended it a changed man. Those bright early spring days marked the first steps of an incredible journey that have seen me put ten months of sobriety under my belt.