The stagnant midsummer heat has settled over the city, which means it’s time for the unbearable chaos of the outdoor concert series at the City Market. As Howie Day and O.A.R. (Of a Revolution) swagger into town, you may question the pair’s appeal. Neither act is hugely popular or truly mainstream. Yet both boast cult followings that will nonetheless lure a swirling mass of sticky youngsters singing all the words to all the songs. Sure, the crowds are thick, the sun is hot and the bottled water is expensive, but maybe the lure of fanaticism is its own reward. And some catchy songs don’t hurt. Day made himself sort of famous with acoustic folk, but he performs a more energetic set of sensible rock with a full band. O.A.R. leans more toward an earthy, jam-band sound. If nothing else, checking out both bands as a member of the semi-obscure-pop-rock fan club is one way to occupy your idle summer.