When we were there the first week in March, fireworks were cancelled both days we were at the park (Friday the 3rd and Sunday the 5th). They started fireworks on the 3rd and stopped about 5 minutes in. They never even started them on Sunday.
I'm a Canadian so I don't really understand the in's and out's of Cali wind conditions...but I can tell you the flag on the top of the Main St. railroad station was not moving either night even a little bit. Both were super clear nights. It was SUPER frustrating! This may be cynical, but it seemed like an excuse for them to save some money.
We still had a great trip, but we planned our days to give ourselves two shots to view the fireworks (they were only showing them Fri/Sat/Sun at the time). Alas, it was not meant to be.