By-elections are held when an existing councillor or Mayor position becomes vacant.  For example, a vacancy can occur when a member resigns, moves out of the municipality, or is disqualified (Section 108, MGA). 

A vacant position requires that a by-election be held within six months of the vacancy unless the vacancy occurs in the last 12 months of the member’s term. Council terms end on December 6th of a general election year (Section 60(1)(b), MGA).

The by-election date must be on a Monday (Section 5(1), Municipal Election Regulations) and notice of a vacancy must be provided to the Minister (Municipal Affairs) within 10 days of the vacancy (Section 5(1.1), Municipal Election Regulations). Municipal by-elections must be conducted in compliance with the MGA, regulations and bylaws (Section 5, Municipal Election Regulations). The by-election day is set by council resolution.

If the Mayor position is vacant, a member of Council can be nominated for Mayor if they resign from their councillor position within 24 hours of accepting the nomination (Section 60(4), MGA). The resulting vacant council position can be filled at a by-election held at the same time as the by-election of the Mayor (Section 60(5), MGA).
When a person is elected or appointed to fill a vacancy, they hold the office until the end of the term (December 6th of a general election year) (Section 62(6), MGA).