Not as hard as sacking a band member you don't want!

Just do it. It's your life.

The best thing to mitigate the blow for the rest of the band, if they are worried about all the gigs lined up and having to do them without you, is to agree a notice period, like when you resign from a job. That way you get to play the Summer gigs, while they recruit for a new member.

The sooner you do it the better, and the longer notice you can give. Maybe you can simply agree to to all the gigs currently booked (and the CD...). They can't complain about that, but you might need a sooner cutoff date than that. Either way, it's their risk then, to take bookings for after the date you've agreed to leave.

Anahata