All schools in Wales will reopen on 29 June, the Welsh Government have 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, Education Minister Kirsty Williams confirmed.
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 pupils who fall into the shielding category will carry on with online learning.
Some teaching unions have criticised the move in Wales, with the biggest union saying its members will be told they do not have to return to school if they feel the risk is too great.
'No educational purpose behind the decision'
David Evans, from NEU Wales, said the measures were "too much, too soon" and there was "little or no consultation" over the additional week at the end of July.
Another union, the NASUWT, said the government did not have "good enough reasons for risking lives."
"The Welsh Government is well aware of the extreme difficulty of social distancing in schools, especially for younger children," said National Official Neil Butler.
"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."
NAHT Cymru said that while the announced plan was the one its members preferred out of the options put forward by the Welsh Government, the intention to get "all" children in "is more problematic".
It added it would have preferred to have prioritised specific year groups.
Ms Williams said she was "surprised" by the unions' criticism and it was not helpful "to have inflammatory language".
"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," she said.
What was Wales' 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 gives schools three-and-a-half weeks to continue preparing for the next phase," she added.
Ms Williams 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?
She 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.
Ms Williams 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.
She 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 supposedly rejecting plans to bring forward the start of the autumn term to August.
So far, the only Welsh pupils who have seen the inside of a classroom since the coronavirus lockdown have been children deemed to be vulnerable or those of key workers, 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.
'I know many will feel apprehensive'
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."
'All school staff should be tested'
Unison, the public services union, said all school staff must be included in antibody testing, not just teachers.
"This approach will not work and will risk the spread of Covid-19," schools lead Rosie Lewis said.
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.
Further education colleges are preparing to reopen for face-to-face learning from 15 June.
What has the reaction been?
Teachers are awaiting the finer detail and logistics of how they are to accommodate the children safely with social distancing in mind.
"I don't think we can ever be reassured it is perfectly safe," said Armando Di Finizio, head of Eastern High School in Cardiff.
"It'd be wrong not to have concerns but it is about mitigating any risks as far as we can but I think it is the right time to go back."
The profession think managing a safe return for pupils will be difficult in smaller primary schools where they "don't have spare space" so "can't spread pupils out".
"If we have different groups coming in, we're going to have to clean those rooms down completely in between each group," said a Cardiff primary school teacher, who would only give her first name, Carol.
"The children will also need their own set of stationary - but we suddenly don't have the finances to buy 500 sets of stationary because they would normally share. It's that kind of practical detail we need."
But the teachers agree that not returning to school until September is "perhaps too long".
"We are there to help young people make sense of the world," added Mr Di Finizio.
He said it would help give children "reassurance, stability, rhythm of life and discipline" as well as "helping vulnerable" and at risk students.
Plaid Cymru criticised the decision, saying it was "too early", while the Welsh Conservatives broadly welcomed the move but said there were still questions to be answered, such as whether assistance with school transport, cleaning and school meals would be provided.