Children in some Welsh counties will return to classrooms for only three weeks when they go back to school after the coronavirus lockdown on 29 June.
The Welsh Government wanted to extend the summer term by a week but unions were concerned a 27 July finish would cause problems for staff contracts.
Conwy schools return for four weeks.
Schools in Anglesey, however, will not reopen as planned after a coronavirus outbreak temporarily shut down a meat processing factory.
A Welsh Government spokesman said: "While we continue to recommend four weeks of check in, catch up and prepare, we acknowledge that ultimately this is a decision for local authorities, who are the employers of school staff."
The difference in end-of-term dates follows a disagreement between the Welsh Government and unions and talks are continuing.
But some councils have made a final decision now, with Wrexham council saying it could not be left "until the last minute".
Education Minister Kirsty Williams wanted the summer term to be extended to 27 July to allow a month of school before the summer holidays.
Some unions questioned the safety and practicality of the proposal and have warned there may not be sufficient numbers of cleaners and teaching assistants to enable schools to open for an extra week.
Unions representing support staff have said they cannot be required to work as their contracts only cover standard term times.
Teachers' unions are also believed to have had concerns about the proposal.
Wrexham council said it put "schools and the council in a very tough position".
"There's no contractual obligation for staff to work the extra week - putting the onus on individual head teachers and staff, which is unfair," it said in a statement.
"It could also lead to inconsistency and confusion, with some schools able to open for the fourth week, and some not.
"We know that many parents will be feeling anxious and uncertain about sending their children back to school, and need to know exactly what's happening so they can make arrangements and feel confident.
"So this isn't something that can be left until the last minute, and it's only fair - to staff, parents and pupils - that we make a decision for the whole of Wrexham now. Not tomorrow, or next week, but today."
In a staggered return, a maximum of a third of pupils will be going back at any one time for "catch ups" a for one or two days of the summer term.
The Welsh Government had said it believed the additional week would be "hugely important in helping schools take a phased approach in supporting all children and young people".