Alexander Schneider will conduct the Curtis Institute Orchestra in a free concert dedicated to Pablo Casals on April 3 to open the Kennedy Center's third annual Spring Festival of free music sponsored by McDonald's.

The week is to be "A Lively Musical Salute to the Hemisphere," with singers and dancers from North, Central and South American countries, and the late cellist is being honored for having made Puerto Rico and international musical center. Eugene Istomin, who is married to Casals' widow, will be the soloist at the Casals concert.

Free tickets for this and for the April 4 concert, which will be a gala with all the week's performers, will be available starting Monday at National Capital Park Service desks in the Kennedy Center. The tickets will be honored up until 15 minutes before concert time, after which the general public will be admitted to the Concert Hall to any seats left vacant.

April 4 through 8 there will be performances in the Grand Foyer, for which no tickets are required. These are scheduled for 10:30 a.m., 12:30 p.m., 2:30 p.m. and 4:30 p.m. On April 9 there will be continuous performances in the Grand Foyer from 10:30 a.m. to 6 p.m.

And on April 6, 7 and 8 at 10:30 a.m. and 1 p.m., "The Great American Band Wagon" will be presented by the Bob Brown Marionettes in the Chautauqua Tent. Tickets for this performances, also free, must be obtained from the park service desks.

The festival is the fifth to be sponsored by McDonald's, the first two having been at Christmans rather than Easter week. A grant of $55,000 was made by the hamburger and restaurant company, and transportation expenses are being contributed by the governements of each performing company.

The United States will be represented by the Billy Taylor jazz trio. Folkloric and popular singers and dancers from other countries will be performing in their national costumes.