From the Commonwealth’s website (PDF file):

First day to circulate and file nominations petitions
January 22

Last day to circulate and file nomination petitions
February 12

First day to circulate and file nomination papers
February 13

Last day for withdrawal by candidates who filed nomination petitions
February 27

Last day to REGISTER before the primary
March 24

Last day to apply for a civilian absentee ballot - primaries
April 15

Last day for County Boards of Elections to receive voted civilian absentee ballots - primaries
April 18

GENERAL PRIMARY
April 22

First day to REGISTER after primary
April 23

Last day for County Boards of Elections to receive voted military and
overseas absentee ballots (must be postmarked no later than April 21)
April 29

Last day to circulate and file nomination papers
August 1

Last day for withdrawal by political body candidates who filed nomination papers
August 8

Last day for withdrawal by candidates nominated by a political party
August 11

Last day to REGISTER before the General Election
October 6

Last day to apply for a civilian absentee ballot
October 28

Last day for County Boards of Elections to receive voted civilian absentee ballots
October 31

GENERAL ELECTION
November 4

First day to REGISTER after election
November 5

Last day for County Boards of Elections to receive voted military and overseas
absentee ballots (must be postmarked no later than November 3)
November 12