It’s supposed to thunder and lightening tomorrow for a concert, how do I know if the concert is still on?

Either check the band's Facebook page for updates from them or call the park office and inquire.

Policy is concerts go on rain or shine. At best, this will be delayed 1 or 2 hours. Storms predicted to subside by 5 or 6PM.

Check with the Fenway Park box office.

I heard they might postpone it till Saturday,

Concerts usually dont cancel. Check the red sox/fenway website or follow on twitter

Call the ticket office