For singer-songwriter-multi-instrumentalists Sara Watkins and Sean Watkins, the Watkins Family Hour has long been an oasis from the rigors of the road; a laboratory where these sibling co-conspirators can try out new material or master beloved cover songs. Boston fans of bluegrass and folk know Sara and Sean from their work as 2/3 of Nickel Creek, and Sara’s work with I’m With Her (A Celebrity Series favorite). Their monthly show at L.A.’s famed venue, Largo, has been hailed as a convivial, communal event where they welcome an impressive array of musician friends old and new. Now this not-so-well-kept secret is coming to the Celebrity Series. The Watkins Family Hour is always full of surprises and one-of-a-kind match-ups; serendipity plays as much of a role as virtuosity. Joining the Watkins is the brilliant folk-bluegrass ensemble Hawktail featuring fiddler Brittany Haas (Crooked Still), bassist Paul Kowert (Punch Brothers), guitarist Jordan Tice, and mandolinist Dominick Leslie.