Local elections for County, District, Unitary and Town & Parish councils occur almost every year in different places. Generally they are on a four year fixed cycle with most councils electing the entire council at once, but some elect one third of the council every year and then have no elections in the fourth year. A few elect half the council every two years.
Here we are only concerned with Principal Authority elections (that is County, District and Unitary Councils). There are far too many Parish and Town Councils to track and many of these are a-political with candidates generally not declaring a party allegiance.