53 Pa. Stat. § 23604
Elections of representatives of the board shall be held annually on the second Monday of December, except that a special election of such representatives shall be held within ten days following the effective date of this act; the time and place for holding of which special election shall be fixed by the existing board in such city. The time and place of holding the annual elections thereafter shall be designated and fixed by the board herein created.
If any member of the board shall resign or be dismissed from the service, or shall change from the classification under which he was elected, he shall immediately cease from being a member of the board.
In case of a vacancy caused by whatever means among the elective members of the board, such vacancy shall be filled by the board at its next meeting for the unexpired term.
53 P.S. § 23604