According to
the basic 7-Line info page, express service is only during rush hour in the expected direction. But,
The MTA Is Your Ride to All Yankees and Mets Home Games
"The
train’s Mets-Willets Point station is right in front of Citi Field on Roosevelt Avenue near 126th Street. Fans can always take either the local or the express
to reach Mets-Willets Point. After weeknight and weekend games, New York City Transit provides special super-express
trains timed to depart after the last out. After leaving the game, these trains make only six stops – 61 St-Woodside, Queensboro Plaza, Court Square, 42 St-Grand Central, 5 Av-Bryant Park, and Times Square-42 St."
According to this, Mets games get special super-express trains back to Manhattan when games end. It also says fans can "always take either the local or express" to get to the game.
I'm pretty sure that for Mets games, and some other events, they run express service on weekends.