It was not until 1919 that New York State legalized Sunday baseball games and what were then called “moving picture shows.” Now, nearly a century later, another never-on-Sunday tradition is coming to an end: The Metropolitan Opera will begin performing Sunday matinees next year for the first time, the company announced Friday.

The Met’s inability to perform on Sundays was rooted in labor contracts, not blue laws. But the unions representing the Met’s orchestra, chorus and several other groups finished ratifying a new contract this week that will pave the way to a change, as the company, facing a worrisome decline in attendance, has realized that audiences find it increasingly difficult to squeeze in lengthy operas on weeknights.

“It’s a real game changer,” Peter Gelb, the Met’s general manager, said in an interview. “It will allow us to attract a new audience.”

Sunday matinees will become a regular part of the Met’s offerings in the 2019-20 season. The following year, the Met will make another radical change in its schedule: It will take a midwinter break in February, when ticket sales are generally at their lowest, and add performances in the late spring, moving the end of the opera season to early June from May. Those spring weeks are currently used by American Ballet Theater; Mr. Gelb said that the ballet would reduce its Met season to five weeks from the current eight.