Callahan's Place was the first bar in the Callahan's series and run by Mike Callahan himself.
The bar is described as being located on Route 25A in Suffolk County on Long Island in New York state. It is marked by a single sign and is set back from the road far enough to be easily missed. Due to the location, walk-ins are rare and it's generally held that, with a few exceptions, people don't find Callahan's unless they're sent or they need to find it. Most stories, in fact, are built around rather bizarre coincidences in the people in greatest distress or need finding their way there, such as having an auto break down just outside, and so on.
Clientelle and Atmosphere
The atmosphere in Callahan's is meant to facilitate enjoyment and relaxation; to this end, there are numerous group activities (on an opt-in basis), such as Punday Nights and Tall Tale Tuesdays. Callahan has a policy of discouraging snoopy questions unless that customer okays them; Fast Eddie, his piano player, is quick with a blackjack and doubles as his bouncer. People can get bounced simply for being too nosy.
Mike likes to euphemistically call being bounced by Eddie (or himself) as an "invitation to the world." In the Place's whole history only one client became so obnoxious that he was thrown out without opening the door first; for some time after, his exit imprint can still be seen in the wood.
This combination of a relaxed atmosphere and the discouragement of prying has an underlying purpose of making his clients comfortable with one another, part of Mike's long-term goal of teaching humans (who seem to desperately need it) empathy, and eventually even telepathic contact (which happens rarely, but saves the day on several memorable occasions). It is also integral to a number of stories, in enabling the clients in greatest need to open up about their burdens without feeling pressured into it. (Also helpful is the Callahan's tradition of making a toast first and then breaking the glass in his specially-sculpted parabolic fireplace.)
Callahan's place has many regulars throughout the series, enumerated elsewhere, and another core of recurring characters, like the "Lucky Duck", who tend to serve as plot devices and drop in and out. These characters go on, after Callahan's original place is blown up (in order to forestall alien invasion), to become the new regulars of Mary's Place--and later still, Jake's Place, in Key West. Being old hands, they tend to take the lead in encouraging those in need to open up, and also to serve as a reliable baseline for their rare telepathic links (typically a catalyst is also needed, such as... oh, twins with above-average psychic ability).
Mike Callahan himself has an unusual degree of empathy for those down on their luck, as witnessed by his "Free Lunch" program for clients in need; this is often funded mainly by those who have had to take advantage of it in the past. As to empathy or telepathy, the true extent of his own gifts is never really explored, but frequently his Place's location seems to prove particularly serendipitous, due to the presence of one particular character (usually a regular) whose unique skills serve to resolve a given crisis. (His wife, Lady Sally's, abilities in this regard are similarly implied yet left ambiguous---though it is often remarked that she seems instantly aware of everything transpiring in her House.)
Significant Plot Developments
For some time, in the Callahan short stories, the plots have a science fictional twist but Mike's status as a time traveler himself goes completely unsuspected. The crises that show up at his door just seem to find ways of resolving themselves, usually with no apparent intervention on his part. Eventually he's forced to reveal himself to one and another of his clients, and sometime later his real purpose here--preserving the Earth from invasion by hostile aliens (of the space type) gets revealed. However, his primary weapon in this battle is not some item or arsenal of future-tech, but the interconnection of his human clients and their determination to keep their world safe. (This showcases author Spider Robinson's "soft science" approach, fairly unique in the genre.)
For example, the aliens' initial scout is dispatched by slipping a Mickey Finn into his drink and thus preventing an automated telepathic transmission, while a rogue member of that race who somehow finds his own way to earth is delayed (and his weaknesses telepathically probed) by a dedicated cadre of Mike's regulars, past and present, who "hey, rube" to his distress call. (Admittedly, however, it's ultimately an atomic bomb hauled to the scene by the bomb squad's own Noah Gonzalez that actually does the creature in--and Callahan's Bar along with it.)
His mission finally accomplished, Mike vanishes to attend to other difficulties in other space-times (or so we're left to assume); in his absence, the regulars decide to continue his traditions and their ongoing "quest for telepathy", eventually opening a new location named after Mike's only daughter, Mary. And so it goes.