Schools are braced for "strike" action by children after tens of thousands of people signed a petition supporting a boycott in protest over exams for six- and seven-year-olds.
The Let Our Kids Be Kids campaign wants parents across England to keep their children off school today, saying they are "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, signed by more than 40,000 people, 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."
In an open letter to Education Secretary Nicky Morgan, published on their website, the Year Two parents behind Let Our Kids Be Kids said they "represent the voice of parents across the country" who "want an end to Sats (standard assessment tests) now".
They wrote: "Please take a long, hard look at this.
"Do you want your legacy to be the confident cancellation of unneeded and unnecessary Sats, showing you are listening to your electorate and the teachers you claim to support ... or the overseeing of a shambolic testing regime desperately unwanted by millions of people to the point that this country saw its first open parent revolt?
"You have the power to stop these tests. NOW. Our children, our teachers and our schools deserve better than this."
Sats are taken by children aged six or seven in Year Two and then again in Year Six, aged 10 or 11, before a third set in Year Nine aged 13 or 14.
Parents are being urged by the campaign to keep their children off school for "a day of educational fun instead" and the website includes posts from hundreds of groups or parents indicating they will take part in another activity rather than attending classes.
One suggestion from Cornwall is for a "fun day of learning at the Eden Project" while other groups of parents across the country were planning nature walks or trips to museums.
Shadow education secretary Lucy Powell said she did not condone the action but claimed ministers had "ridden roughshod" over the concerns of headteachers and parents about Sats.
Ms Powell said a series of policy changes had caused "chaos" for schools, with 82 primary assessment documents published, updated or clarified on the Standards and Testing Agency website since the start of the academic year - an average of one every other working day.
The shadow education secretary 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.
"This is on top of the calamity of putting the Key Stage 1 Spelling and Grammar test online early, forcing the paper to be cancelled, rendering the work of children, teachers and parents null and void.
"Ongoing assessment is really important to help teachers and parents support their children's education and to close any gaps in knowledge so all children can do well at school.
"However, the way this Government has handled the assessment regime means that parents and teachers are lacking confidence in school tests and the assessment system is being undermined.
"Whilst I don't condone children being taken out of school, the blame for the lack of confidence we are seeing in these tests lies firmly with this Tory Government and education ministers, who have ridden roughshod over the concerns of headteachers and parents over the constant chopping and changing of the exam and assessment system."
The Education Secretary warned that missing school even for a single day would be "harmful" and called for those behind the "damaging" campaign to reconsider their actions.
In a speech 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.
"Children should only ever be taken out of school in exceptional circumstances and we'd urge the organisers of this campaign to drop their plans because it simply isn't fair on children to deprive them of a day of their education."