Sometimes, in music, it's best not to plan too much.

Jim Weider - leader of the profoundly great roots band PRoJECT PERCoLAToR, longtime guitarist for The Band and trusted sideman for acts ranging from Bob Dylan and Graham Parker to Los Lobos and Mavis Staples - couldn't agree more.

After all, it was a very casual idea that blues harp legend James Montgomery would sit in at a PRoJECT PERCoLAToR gig last year in the theater of the Olde Mistick Village Art Cinemas.

And it was completely unexpected that a by-chance soundboard recording of the gig was so infectious that Weider decided to release it as an in-concert CD.

But those are the facts behind the brand-new "Jim Weider's PRoJECT PERCoLAToR Live at Olde Mistick Village Performing Arts Center - Featuring James Montgomery."

It's admittedly a lengthy title but, at the same time, it's representative of all the odd elements that fell into sync for it all to happen.

In celebration, Weider and PERCoLAToR - along with Montgomery - return to the Olde Mistick hall tonight to play a release party. Copies of the CD will be available for sale for $20, and the musicians will sign them.

"It was James' idea to even come down that night," Weider remembers of last year's show. "I've known him a bit from the circuit and he'd sat in with us a few times. He always adds such excitement and he's such a gentleman. I told him, 'Fantastic! Come on!'"

As it turned out, they had to scramble to make soundcheck. "We were running very late," Weider says. "One of us had to travel from Canada; another came from Philly. Then James showed up, so we ran down a few blues standards and showed him a few of our songs, basically saying, 'And this is how they end' - then went with (Olde Mistick Cinemas owner) Bill Dougherty to the Steak Loft for dinner."

Unnoticed throughout the evening was that the band's soundman had brought a DAT machine and recorded the performance. Afterwards, Weider was called over to hear the recording - and was excited by how good it sounded. Ultimately, it was decided it would make a spirited and representative live album.

"We weren't remotely thinking in those terms," laughs Weider. "We certainly didn't plan it, which is exactly what I like about it. There are some imperfections; you can hear my tuner slipping at one point, and there's a bit of dissonance, but I just left it in, you know? It was part of a very good show and it keeps it real."

Also left on the recording is Weider, between songs, giving effusive thanks to Dougherty, whose photo is included on back of the CD jacket.

"We were excited to have Bill part of the album," Weider says. "He's a great person and it's a great venue with wonderful sound. I told him, 'You're immortal now, Bill.' At least in our small way."

While tonight's show will roughly follow the set list from last year's show - complete recitations of the band's "Pulse" and "PERCoLATOR" albums - there will always be surprises. Montgomery has an encyclopedic knowledge of the blues, and Weider himself is a devotee of the form.

"I've played and loved so many musical styles," Weider says. "But the electric country blues has always been my thing. Muddy Waters, Mississippi John Hurt, Taj Mahal, Jesse Ed Davis ... Those guys had a big influence on everything I do. It's a starting point for all roots music."

Jim Weider's PRoJECT PERCoLAToR, 8 tonight, Olde Mistick Village Performing Arts Center, 17 Coogan Blvd., Mystic; CD release party with James Montgomery; $20; (860) 536-4227, oldemistickflicks.com.