Around the weekend of July 14th is when Radio 1 usually hit Magaluf with DJ's and people like Trevor Nelson. The last two years have been like that anyway.
DJ Sammy is usually there every fortnight too and then you get people like Judge Jules and Dave Pearce once a month or so. Just depends, there is no sure fire way of finding out who is on unless you pick the flyers up there or listen to Radio 1 as there website is never updated!
Themed nights though, downstairs in Millenium they usually have one water/foam/beach party a week with some other stuff like Haiwaan contests with DJ Gotti.