More than half a million families are discovering which primary schools their children will attend, amid a growing places squeeze in parts of England.
The day will bring relief for many, but others will be disappointed at learning they have missed out on a first-choice school.
It comes days after council leaders said two in five local authorities would have too few places by 2016.
Some schools have been maximising their capacity by building extra classrooms.
Pressure is growing, particularly in schools on London's fringes and in cities such as Leicester, Nottingham, Reading, Bristol and Peterborough.
And it is likely that more families will miss out on their first-choice school as the places crisis continues to bite.
'Balkanised system'
The crisis has been precipitated mainly by a booming birth-rate, partly by immigration and by families moving specifically to be near popular schools.
The demand for school places has risen steeply in Harrow, which has some very good primary schools. It was predicted to be 12% over capacity by this September but the council said it had worked really hard in its strategy to ensure there are enough places for Harrow children this year.
A spokesman said the crush Harrow was seeing now was very likely to affect other areas around the country soon.
Labour and the National Association of Head Teachers also blame the coalition government's free-school policy, which has allowed some new schools to be opened in areas that already have surplus places.
At the same time, the ability of local authorities to plan for population surges has been reduced by regulations requiring any new schools to be either an academy or a free school, rather than a council school.
NAHT general secretary Russell Hobby said: "Since 2011, the powers of local authorities in planning school places have been significantly reduced without an alternative system to take their place. We have a balkanised system with authorities, academies and central government taking decisions in isolation."
'Reaching the limit'
He added: "There is a desperate need for long-term planning that spans all sectors. With the massive increase in pupil numbers and over-stretched budgets, we cannot afford inefficiency and conflict."
The Local Government Association said the escalating places shortage was one of the key areas that needed to be tackled by the next government.
Earlier this month, it said schools were "reaching their limits and could soon run out of space and money for extra places" and there would be a need for an extra 880,000 pupils at a cost of £12bn.
The Conservatives blame Labour for the problem saying the party "cut over 200,000 primary school places - and even ignored official warnings to provide extra school places after a baby boom".
A spokesman added: "The Conservatives have created over 400,000 school places. There are fewer children in overcrowded primary schools and, most importantly, one million more children in good or outstanding schools since 2010."
'Raise standards'
Labour points out that under the Conservative-Lib Dem administration the number of infant children in classes with more than 30 pupils has more than trebled from 31,265 in 2010 to 102,615 in 2015.
Shadow education secretary Tristram Hunt blamed the Conservatives for spending "hundreds of millions of taxpayers' money on a few free schools in areas where there are no shortages of places".
Liberal Democrat schools minister David Laws said: "It's astonishing that Labour and the Conservatives won't properly fund these children. It will be impossible to provide a place for every child and raise standards unless we protect education budgets.
"Every parent getting an offer today wants to know their child will be attending a great school. Protecting education means we can put a qualified teacher in every classroom, expand early years education and support the children who need extra help."
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