All schools in Wales will reopen on 29 June, the education minister has said.
They will be open to pupils from all year groups for limited periods during the week, with only a third of pupils in school at any one time, Kirsty Williams said.
Schools and councils will make their own decisions over managing the return.
The summer term has been extended by one week to 27 July, and the autumn half-term holiday will be stretched to two weeks.
Parents who choose not to send their children to school will not be fined and children who fall into the shielding category will carry on with online learning.
Some teaching unions have criticised the move, with NEU Cymru saying its members will be told they do not have to return to school if they feel the risk is too great.
David Evans, from NEU, said the measures were “too much, too soon” and there was “little or no consultation” over the additional week at the end of July.
Neil Butler, of NASUWT, said the reasons were “not good enough”.
“The Welsh Government is well aware of the extreme difficulty of social distancing in schools, especially for younger children,” he added.
“The minister has also admitted that this is for a ‘check in’ and ‘catch up’ so there is clearly no educational purpose behind this decision.
“These are not good enough reasons for risking lives.”
But NAHT Cymru said it was its members’ preferred plan, though the intention to get “all” children in “is more problematic”, it said, preferring prioritising specific year groups.
Ms Williams said she was “surprised” by the unions’ criticism and it was not helpful “to have inflammatory language”.
She told journalists: “I would do nothing, and I have done nothing from this entire period when we closed schools for statutory purposes, to do anything to risk anybody’s lives.”
What was Kirsty Williams’ schools announcement?
Year groups will be split into smaller classes with staggered starts, lessons and breaks. It is expected this will mean, at most, a third of pupils present at any one time, though schools may need time to reach this level.
All children will have the opportunity to “check in, catch up, prepare for summer and September”, Ms Williams said.
“My announcement today gives schools three-and-a-half weeks to continue preparing for the next phase.
“We will use the last weeks of the summer term to make sure pupils, staff and parents are prepared – mentally, emotionally and practically – for the new normal in September.”
She said social distancing would be a challenge for the youngest children “but we have experience of dealing with this in the hubs that have been operating to date” and good practice had been learnt.
Why are Wales’ schools reopening before September?
Ms Williams said the Welsh Government was “moving very cautiously” by limiting the number of children in classrooms at any one time and also the number that can be at school at the same time.
She said it was important to ensure “equity” by giving all children the chance to “catch up” and prepare with their teachers ahead of the summer holidays.
Ms Williams said she had been “hugely heartened” by headteachers “coming forward with very practical plans” on how they can “move to the next phase of supporting children and families in their learning”.
By doing this before September, she added, “it gives us a natural firebreak to reflect on those experiences” before the next term.
The Welsh Government had been considering two options – to reopen this month or wait until after the summer holidays – after rejecting plans to bring forward the start of the autumn term to August, according to unions.
So far, the only Welsh pupils who have seen the inside of a classroom since the coronavirus lockdown have been children of key workers, or those deemed to be vulnerable, who have been attending school hubs to enable their parents to continue doing their vital jobs.
The Welsh Government will issue further guidance for councils and schools next week.
Ms Williams said the start date would mean one full month of the test, trace and protect system being in place – which tracks people who have been exposed to the virus and asks them to self-isolate.
Teachers will be a priority group for the new antibody testing programme, which tells people if they have previously had the virus and have developed antibodies.
“As we continue to keep Wales safe, this approach will be critical,” she said.
“The evolving science suggests that warm weather and sunlight gives us the best opportunity to ensure more time in school.
“Waiting until September would mean almost half a year without schooling. That would be to the detriment to the wellbeing, learning progress and mental health of our young people.
“This is and has been a worrying period for us all. I know that many will feel apprehensive. We have not rushed this work and this decision.
“I am also convinced that it is only by returning to their own school that we will see increased attendance from our more vulnerable and disadvantaged children.”
The government will also issue guidance for further and higher education institutions next week, and for childcare providers to increase the number of children in attendance alongside schools.
It is publishing a paper about its latest understanding of the virus with respect to children and education.
Children in England started back on Monday, although some schools have not reopened and attendance has varied.
In England, reception, Year 1 and Year 6 classes have been the first groups to return.
Schools in Scotland and Northern Ireland will begin to return in August.