Celtic music is played all over the world, on stages  before capacity crowds in venues of all sizes and settings. But however you dress it up, and wherever you take it out, the music sounds most at home in an honest-to-goodness session, whether in a pub or in someone's living room.

And it is this very essential element of Celtic music that will be celebrated in the June 8 edition of BCMFest's Celtic Music Monday series at Harvard Square's Club Passim. "Trad to the Bone" will  feature some of Greater Boston's best session musicians from the Irish and Cape Breton traditions: Mainstays such as Matt and Shannon Heaton, George Keith, and James Hamilton will represent the Irish aspect, while Doug Lamey will lead a group of Cape Breton players. No matter how experienced or accomplished a musician is, a good session can offer a number of benefits: inspiration, education, socialization, and relaxation. Those who regularly perform in concert settings say they often find a session as a means of recharging their batteries, and an opportunity to just play music without having to worry about things like stage presence or sound systems.

"It's a place where we can all have fun sharing a common passion, whether it's around the corner from your house, or 3,000 miles away," says Keith, a fiddler who has performed with Robbie O'Connell and  Aoife Clancy and appeared in "Christmas Celtic Sojourn." "Irish music crosses remarkable boundaries.  I've made many fast friends from hugely different walks of life that I would never have met were it not for Irish music.  The ability to play traditional music is a ticket to nearly anywhere, and the session is the train station."

Of course, everyone has his or her own idea of what a good session is, adds Keith, who's been a regular at The Burren, The Druid, and the Brendan Behan sessions, among others. For some, it might mean a crowd of 30 musicians or moreplaying tunes everyone knows, whereas others prefer a small gathering (half a dozen or less) in which the repertoire is less widely familiar.

"In the end, though, everybody wants to get something out of going, whether it be a new tune or three, or just having had a fun night out.  But when the stars are aligned correctly, a good session can lift your spirits just as much as any great concert or hard-won sporting event, leaving you skipping home in the wee hours of the morning."

Part of what makes a session attractive for Hamilton, a California-born flute player whose resume also includes appearances with O'Connell and Clancy as well as at BCMFest, is the combination of skills, experience, and styles among the players. While in some situations the differences may be too varied, he says, when it works the result is a joy to behold.

"The music at the session is raw and visceral," says Hamilton, who sits in at the Burren, Druid, and Behan sessions, among others. "To see the level of excellence at which it's played, by people who may not be even remotely professional, is wonderful. You can't help but be drawn in."

Keith and Hamilton have their own checklists for what defines a "good session." Keith likes a small-group size (ten or less), in which the  musicians listen well to each other and adapt as necessary to make  the music "sound, and feel, as good as possible for everybody there."  Other key facets, he says, include a common sense of rhythm among the players, and the selection of tunes - "all the ones you love, none of  the ones you hate. Some of the best sessions I've been to involved playing dozens of tunes I used to love, and hadn't thought to play recently. Discovering an old favorite tune is a bit like meeting an old friend after being apart for a year."

Hamilton looks for the right mix of instruments, not only in terms of  sound but repertoire: "If there's too many fiddles, chances are the
flute, pipes or other instruments will know only so many of the tunes," he explains. "If there are too many flutes, then all the tunes will be in G." He also prefers "an even level of musicianship among the players so there's not too much frustration"; a good  accompanist, he adds, "is gold - somebody who can give you perfect  rhythm, who knows the tunes and isn't just playing chords."

Is it possible for non-musicians, or even people with little or no  previous exposure to Celtic music, to enjoy watching a session? Hamilton thinks so - it has to do, he says, with not only listening to the music but picking up on certain nuances.

"How close are the musicians sitting next to one another? Are they paying attention to one another, picking up on what's happening with a tune and where it's going?  If they all know each other well, and  the tunes that they're playing, most of the time they only have to  look at one another to switch to another tune or perhaps do a variation on the tune they're playing. It's fascinating, and enjoyable, to watch that kind of communication.

"That's why, in my experience, a musician who's a great performer isn't necessarily someone I want to play with in a session. A performer has to know how to 'switch off' to be a good session player, because in a session it's not about standing out from
everyone else; it's about blending your ability with the other  musicians so that together you produce this incredible music.

"Ironically, though, when you see a group of session musicians who are doing that, it's as exciting as any performance."

Admission to BCMFest Celtic Music Monday is just $12, $6 for members of Club Passim, WGBH and WUMB. To reserve tickets, go to clubpassim.org.

BCMFest Music Cruise June 14 -- One of the most popular BCMFest events, the fourth annual BCMFest Music Cruise out of Gloucester Harbor, will take place Sunday, June 14, from 5:30 to 8 p.m. Come ride the schooner Thomas E. Lannon with a "crew" of BCMFest  performers, including singer Michael O'Leary, fiddler Doug Lamey,  accordionist Susie Petrov, harpist Carol McIntyre, piper/whistle  player/singer David de la Barre, banjo/mandolin player Steve Levy and guitar/bouzouki player Lin Swicker. Your donation of $50 (most of the  proceeds go to benefit BCMFest) will land you a spot on deck of the  65-foot schooner, where you'll spend the evening cruising around  Gloucester Harbor while listening to great tunes and singing along  with a good sea chantey or maritime song.

This event has sold out for all three years it has taken place- so  don't be left behind on shore! For information and reservations, go to the Thomas E. Lannon Web site [schooner.org] or call 978-281-6634.

Performer application deadline July 1 -- Just a reminder that the deadline for performer applications for  BCMFest 2010 is July 1. It's easy: Just go to bcmfest.com, download an application. and follow instructions. If you have a question about BCMFest, or you'd like to sign up for  the BCMFest e-mail list, contact us at bcmfest@gmail.com.