Good question, it depends on what sort of business you have; sportsbook are always quiet over the summer when their's no football on, of course their are many other sporting events to bet on but football (soccer) is the major revenue stream for all sportsbooks. Except when we have tournaments like the World Cup and European Championships.
Poker and Casino businesses are pretty steady throughout the year. Pokerrooms peak right before big tournaments such as the WSOP, WPT, EPT, etc... because all online poker sites offer saettelites