Shoppers laden with packages milled through the music-filled street clutching balloons and munching on free popcorn yesterday, all part of a Patriot's Day sale--the first springtime Sunday opening of the Harvard Coop.

Although potential purchasers streamed through the open doors, Coop officials said that Sunday sales at the Coop would not be a regular event, despite the repeal of the ancient Massachusetts "Blue Laws," which until last month prohibited Sunday retail sales. With employees receiving time-and-a-half pay for Sunday hours, and no reliable information on Sunday profit returns yet available, officials remained skeptical about the benefits of the change.

"We're still not convinced that Sunday will be a profitable business day," said Store Manager Allan E. Powell yesterday.

Coop houseware buyer Marguerite A. Luhrs said yesterday that Sundays may generate considerable profits. "Cambridge will be especially successful because it is an active place where people like to be out and around."

Although still in the planning phase, a sale during Commencement week will probably be the next Sunday opening, Powell said.