This is a town festival in the proper sense: run by Arenys folk, for Arenys folk, and you're simply welcome to stand in the crowd. Over three days each August the old streets and the dry riverbed fill up with macips in traditional dress, the morning collection winding house to house, the Goigs sung to Sant Roc, and the local Ballada danced out in the square. Expect a giants' parade after dark, a ruixada (a soaking water blessing, so stand back unless you fancy it), and a street performance called La Pesta that drags you into the town's plague-haunted sixteenth century.
It's free and unpolished, which is rather the point. Come for the religious service and town vow if you want the heart of it, or just wander down in the evening for the bands and DJs once the formal bits wind down.