Some 5000 alumni and their families began returning to Cambridge yesterday afternoon for the traditional fiveday program of alumni celebrations and ceremonies which will be climaxed by Commencement on Thursday.

The Twenty-Fifth Reunion Class of 1934 met at a reception in Harkness Commons yesterday. Other classes having formal reunion events during the week are 1904, 1909, 1914, 1919, 1924, 1929, 1939, 1944, 1949, 1953 and 1956.

University offcials estimate that there will be 15,000 people in Cambridge by Thursday, to witness the awarding of academic and honorary degrees.

Registration will continue for the Class of 1934 today at the Union, and the Class will attend Memorial Service at noon at Memorial Church.

Today also, the alumni will join students at the annual Phi Beta Kappa Parade and Exercises in Sanders Theatre at 11 a.m. I. A. Richards, University Professor, and Henry A. Murray, professor of Clinical Psychology, will deliver the poem and Oration.

At night the alumni will take over Symphony Hall for a special concert of the Boston Pops.