(Pictured: children joined teachers on a picket line in Sherwood, Nottingham)
The council estimates that more than 50 primary and secondary schools are affected by the action, with at least 18 closed altogether.
NUT teacher Sheena Wheatley told UnionNews: “We are concerned about teachers’ ability to rest and recuperate during the summer as well as children’s ability to do that.
“But we’re also concerned about the way such a major change is being forced through.
“The council says it’s a consultation but repeatedly says what the outcome of that consultation will be.
“And when we’ve challenged them on it, their response has been to issue a notice that they’ll dismiss us if we don’t agree to vary our contracts.”
Several local authorities in England have toyed with the idea since it was first mooted in a study in 2000 for the Local Government Association (LGA).
Unions say members already spend longer in the classroom than in many European countries and that school holidays are also much shorter.
Nottingham City Council officials argue that the change will help provide continuity and ease childcare issues in the summer.
NUT has organised a mass rally in the city centre this lunchtime.
UnionNews will post a film report on the strike later.