Roundabout's comfortable comedy of a family reunion at Christmastime; The Holly and the Ivy Play by Wynyard Browne. Directed by Lindsay Anderson.

The Roundabout Theater Company is getting a head start on the holiday season by decking one of its halls with ''The Holly and the Ivy.'' The 1948 sentimental comedy by Wynyard Browne concerns a Christmas family reunion at a Norfolk, England, vicarage. It is a sturdy, cozy, comfortable sort of play. Rather than taking sides in the conflicts that threaten the Rev. Martin Gregory (Gwyllum Evans) and his immediate relatives, the author guides his basically decent individuals to find the self-motivations that lead to happy resolutions.

The question posed at the outset of the comedy is whether the recently wi-dowed parson's daughter Jenny (Jennifer Harmon) shall be free to marry her Scots admirer, David Pater-son (Gerald Walker), and go with him to South America. The door appears to close shut on Jenny when her fashion-writer sister, Margaret (Pamela Brook), makes it clear that she has no intention of giving up the glamorous London scene for life in a Norfolk vicarage keeping house for Daddy.

The play achieves the aims of its plot through the behavior and attitudes of its characters. Everyone is essential to the central situation, including cranky Aunt Bridget (Helen Lloyd Breed), romantic Aunt Lydia (Betty Low), and bluff cousin-in-law Richard (Thomas Ruisinger). Typical of this well-fashioned, old-fashioned kind of dramaturgy, even the incidental characters have their points to score and their contributions to make.

The strains and tensions beneath the facade of Yuletide jollity begin emerging when Mick Gregory (Frank Grimes) and Margaret return in a drunken condition from their supposed visit to the local cinema. The incident - and its scandalous potential, given the time and small-town locale - lead to the moments of truth-telling to which ''The Holly and the Ivy'' has been leading. That the Rev. Mr. Gregory and Mick are based on the playwright's father and himself adds autobiographical reinforcement to the spirited debate in which the two men struggle to argue their points and sort out their differences.

Because director Lindsay Anderson has avoided any hint of condescension toward the play, its period, or its people, ''The Holly and the Ivy'' rejoices in its own particular warmth and genuineness. It possesses the disarming sweetness of a Christmas carol. As the Irish-born vicar, Mr. Evans expresses the kind of well-meaning paternalism and ministerial zeal that can somehow ignore the needs and feelings of those closest to him. And those closest to him are admirably played by the Misses Harmon and Brook and Mr. Grimes.

''The Holly and the Ivy'' provides a winningly attractive opener for the Roundabout's 17th season. The hospitable production was designed by Roger Mooney (setting), A.Christina Giannini (costumes), and Ronald Wallace (lighting).

The Christmas entertainment also inaugurates the Susan Bloch Theater (formerly Roundabout Stage Two). The intimate little Off Broadway playhouse was renamed in honor of the distinguished press director who served this and other performing-arts institutions with rare diligence, devotion, and professionalism.