25 Pa. Stat. § 2778
Whenever a vacancy shall occur in either house of the General Assembly whether or not it then be in session, the presiding officer of such house shall, within ten (10) days after the happening of the vacancy, issue a writ of election to the proper county board or boards of election and to the Secretary of the Commonwealth, for a special election to fill said vacancy, which election shall be held at the next ensuing primary, municipal or general election scheduled at least sixty (60) days after the issuance of the writ or such other earlier date which is at least sixty (60) days following the issuance of the writ as the presiding officer may deem appropriate: Provided, however, That should the Governor after the issuance of the said writ of election advise the presiding officer that the General Assembly will be called into extraordinary session prior to the date set for such special election, the presiding officer may countermand the writ theretofore issued and shall issue a new writ of election, fixing therein such earlier date therefor as is deemed expedient, but which shall not be less than sixty (60) days after the issuance of said writ: Provided further, That if the vacancy shall occur less than seven (7) months prior to the expiration of the term, a special election shall be held only if, in the opinion of the presiding officer, the election is in the public interest.
25 P.S. § 2778