all 5 of my bands do the thing where in January of each year, we coordinate when our vacations/work travel is.
we use texting to communicate and bs, and I keep all of the dates in my iphone...the booking person in each band use their own calendar system...in the metal and, our guit player books, and she still uses an old school paper calendar that hangs in the practice area...
After that, it is then just first come/first served as far as I go. As soon as band A books a gig, I inform bands B, C, and D about it. This system has worked well for me for 20 plus years. There have even been quite a few times where I play with the jazz band on a Friday from 6-9pm, and then shoot to the dive bar to play with the metal band at 11.
It also helps to know the general "feel" of how each band likes to gig, and when they will look to book shows:
My metal band likes to book shows between 6-8 weeks apart; the surf-punk and plays once a year (our bass player does not like to play out much anymore..
) the rockabilly band mostly plays in the summer; the jazz and plays mostly in the winter; my wind ensemble does a Christmas concert, Early Spring concert every year, and 4th of July every other year;