Here's something that happens virtually every year and irks me. This past week, I received a call from a big band in town for March 24th. I happen to know that it is Megilah night, and I find it disingenuous that they are calling musicians and try to get them to commit to a Purim gig without mentioning that salient fact. I've heard from other musicians who have accepted such gigs in the past without checking their Hebrew calendars and then have felt stuck because they don't want to -- or couldn't afford to -- cancel on a big office, even though they would never have accepted the gig had they realized what it was.
There are many reasons why musicians may not want to accept a sideman gig that night.
Here are a few:
1) If the musician is a bandleader or is capable of performing solo, (i.e. guitarist, keyboardist, or vocalist who performs with backing tracks) they may prefer to book their own gig.
2) If the musicians leads their own band, or regularly plays for another band, they may prefer to hold the night for that group.
3) If the musician is Jewish, there may be logistical issues with hearing Megilah and being on time for the gig.
4) Some musicians don't want to work on Purim because they view it as family time.
5) Some musicians don't want to play certain venues like many yeshivos because the gigs are unpleasant and there is a risk of injury/equipment damage when playing in a roomful of out-of-control drunks. (Some musicians love these gigs.)
I think that if a band disingenuously tricks a musician into accepting a Purim gig, the musician is within their rights to cancel them when they discover the deception.