In my experience, flying Easyjet to Spain in August, you tend to get cheaper flights on Tuesdays and Thrusdays. Also, I tend to book 6/7 months in advance, but still pay about £170 per person from Gatwick (return).
Monarch flying Gatwick to Majorca 7/8/12 return 21/8/12 is currently £150 return.
Easy won't have their prices up until the last week in September.
Without knowing where you're flying from it's tricky, but I would suggest looking next month on Skyscanner's site to give you an idea of who's got the best offers from your ideal airport. Then go to the airline which shows the best deal.
Certainly living where I do (local airport Malaga) I tend to leave here on a Wednesday or Thursday for Gatwick and return on a Monday/Tuesday/Wednesday for best prices. Often, to here anyway, the late afternoon flights out of Gatwick which get me home for around ten pm are generally the cheapest. Flying out of here on either the mid morning or late evening flights are also the cheaper options. I should imagine it's much the same for Mallorca too.
Definitely have a look at Skyscanner - if only to give you a good idea of who flies there and from where. It's sometimes cheaper to book through the links on their site, but it's also worth pulling up the airlines involved on screen to check - you can get more scope with days here and there IYSWIM