The Brattle Theatre has taken a busman's holiday from its Shakespearean tragedy with two modern tours de force which provide a highly entertaining, if at times puzzling, program. Both Christopher Fry's "A Phoenix Too Frequent" and Thornton Wilder's "The Long Christmas Dinner" have their messages and their morals. Fortunately, however, these are practically unintelligible when surrounded by a superbly fantastic plot in the first play, and then kaleidoscoping ninety years in less than and hour in the second.

Mr. Fry's flight into the world of fantasy may be a disappointment to those who remember him for his more serious work, and even for the fanciful "Ring Around the Moon." For "A Phoenix Too Frequent" does not contain the well-constructed plot and incisive characterization which marked much of Fry's early work. It does, however, have a great deal of situational humor and a masterful contrast of high and low dialogue.

The story takes place in an underground tomb where a rather idealistic young lady (Jan Farrand) and her maid (Nancy Marchand) have decided to starve themselves into Hell in order to keep Miss Farrand's late husband company. A handsome young corporal drops in, falls in love with Miss Farrand, and distracts both ladies back to a more normal interest in life. Miss Marchand is particularly engaging as the brazen ladies' maid whose carthy comments create an amusing foil to the high-flown sentiments of the two lovers.

Wilder's "The Long Christmas Dinner" is a mighty spooky play, and comes as a shock after the frothiness and gossamer of the one preceding it. While it is dedicated to the simple theory that the ups and downs of human existence are pretty small matters after all, it is hard to view death with Wilder's objectivity. Three generations of a family are born in the play, have their Christmas dinners on stage, and die by walking through a door covered with black crepe paper. These deaths mount up after a while.

But the play is an effective one, largely because of Peter Temple's direction. The settings by Robert O'Hearn are attractive, and the costumes managed to convey successfully the different eras of the play's span. Jerry Kilty was outstanding as the second generation head of the family, while Nancy Marchand, in her touching portrayal of the aging cousin, Ermengarde, did a complete turnabout from her first hour on stage.