If a concert is cancelled due to weather, how soon before the concert would it be called off?

It depends on the weather and the artist, usually

What if it snows or falls below freezing?

Last year Muse came on and it was pouring, they stopped it only due to lightning for about an hour n half..we all went to our cars and grocery for beer came back parked and and went back in...they finished the set.... It really depends on artist but most shows are no refunds and they play rain or shine.