All dressed up for summer.Seattle’s biggest indie-folk sextet The Head and the Heart have been spending this week’s heat wave — coincidentally some of the few days they’ve been in town since they launched their perpetual tour in 2010 — churning out a holiday single. And they’re doing their best to not let the heat get in the way of their holiday cheer.”(Bassist Chris Zasche) actually brought in a Christmas sweater in the spirit,” vocalist Jonathan Russell said over drinks in a sweaty Fremont bar yesterday, “and that lasted about an hour.”The song — the name and intended project will be announced in the fall — is a jazz standard that’s giving the band a chance to do some experimenting in the studio. “I’m like, crooning a little bit,” Russell says, “it’s kind of fun.””I was worried it was going to be an insufficient attempt to do a jazz standard,” vocalist/guitarist Josiah Johnson says. “And I think that we screwed around with it enough that it’s really rewarding.”The band launches what is likely to be their last headlining tour in support its self-titled, 2010 debut at the Paramount on September 15, and will stay on the road through the fall — though maybe not long enough to warrant busting out a holiday single.”I kind of, like, half wish that we would (play it),” Johnson says. “Maybe at the right moment.”