https://www.npr.org/2020/10/19/922411176/supreme-court-rules-pennsylvania-can-count-ballots-received-after-election-day
"In its ruling, the Pennsylvania Supreme Court said that ballots could be counted if they were received by 5 p.m. Nov. 6, as long as they were mailed by Election Day, Nov. 3.
It also said that ballots without a postmark would "be presumed to have been mailed by Election Day" unless there was strong evidence to the contrary."