Schoolchildren across England are "on strike" in protest at controversial tests for six- and seven-year-olds.
The action comes as more than 40,000 people have signed a petition supporting a boycott of Year 2 Sats by teachers.
The Let Our Kids Be Kids campaign has organised the day of action in protest at children being "over-tested, over-worked and in a school system that places more importance on test results and league tables than children's happiness and joy of learning".
The petition added: "We want our kids to be kids again and enjoy learning for learning's sake, not for Ofsted results or league table figures.
"Bring back the creativity and the fun – say goodbye to repetition and boredom."
Sats are taken by children aged 6 or 7 in Year 2 and then again in Year 6, aged 10 or 11..
Ben Ramalingam, from Brighton, is keeping his five-year-old son off school in protest at what he says is a hothouse culture that has made young children stressed.
He said: "Our kids are being left disengaged and stressed. Kids who previously loved school are now refusing to go.
"There are a number of people who are saying this has the potential to turn into not just an educational crisis, but a mental health crisis."
He said there is no proof that testing improves educational attainment, and stressed that many countries who take a more relaxed approach to assessments perform better in international education league tables.
He added: "There is an experiment being run on our children and there is no proof it works. It is really inappropriate and, I think, unethical to do it.
"We are concerned parents taking a stand, we don't want our kids to be stressed out by the time they become teenagers because they have been inappropriately taught. Our children are being pushed towards rote-based learning. It is like something out of Charles Dickens.
"We feel we need to send a message to Westminster and Whitehall that enough is enough, they need to see that the actions they take have a highly negative impact."
Jane Clout, a grandmother from Brighton, is keeping her two grandsons, aged 6 and 8, off school in protest at the tests.
She said: "Some of my grandson's friends have been in tears at night, some of them have been having tummy aches which is a sign of stress. It is just so sad.
"I'm a grandmother and I sent my children through the state system in the late 1970s and early 1980s, and when I first started taking these boys to school I was struck by how primary schools have improved.
"But this is like going back to the 1950s."
Children's laureate Chris Riddell is expected to be among concerned parents meeting in Preston Park for a day of activities during the protest, and parents and children are organising fun and educational activities all over the country.
Former head teacher Kit Messenger, who quit the profession after 23 years after claiming schools are "factory farming" children, told ITV's Good Morning Britain that she believes in pushing children on their reading, writing and grammar.
But she said this should not be at the sacrifice of other essential workplace skills such as resilience and interpersonal abilities.
She said: "If you look at the quantity that is required for teachers to teach in order for children to be successful in the papers now – it is to the sacrifice of everything else.
Ms Messenger said tests "do not necessarily tell you where children are" and "there is not time within the curriculum" to prepare children for the tests and teach them other skills.
Shadow education secretary Lucy Powell said that she did not condone the action, but claimed that ministers had "ridden roughshod" over the concerns of head teachers and parents about Sats.
She said: "This government is creating chaos and confusion in primary assessment in schools, with a huge number of changes to Sats specifications since children started school last September."
Education secretary Nicky Morgan warned that missing school even for a single day would be "harmful" and called for those behind the "damaging" campaign to reconsider.
In a speech at the NAHT heads' union annual conference on Saturday, she said: "To those who say we should let our children be creative, imaginative and happy: of course, I agree, both as a parent and as the education secretary.
"But I would ask them this: how creative can a child be if they struggle to understand the words on the page in front of them? They certainly can't enjoy them.
"What are the limits placed on a child's imagination when they cannot write down their ideas for others to read?"
Schools minister Nick Gibb said: "These tests are vital in helping schools to ensure that young children are learning to read, write and add up well.
"The truth is, if they don't master literacy and numeracy early on, they risk being held behind and struggling for the rest of their lives."