The advantage of being older than most of you youngster parents is I have a much longer memory. I can recall state schools in the 1960's and early 1970's having long summer holidays too - exactly the same as the independent schools.
I cannot though for the life of me recall when it changed but I have a gut reaction that this was in the mid 1980's - around the time of LMS and Baker's first National Curriculum implimentations.
But lets face it dearies, all you want is a school to babysit your kids for you ( harsh but true) because you have work committments. Thats why holidays have got shorter in state schools. Indeed with that and breakfast clubs and after school mentoring and holiday revision classes etc ( summer school) most state schools seem to be open much longer than independents.
This is not to do with education. All those bright ideas of independent schools giving longer hours tend to be false. In fact a standard school day in an independent is much shorter - what you see are the extra hours provided in additional activities . That has always been the case. Yes we had Saturday morning schools but that was offset by a mid week afternoon off.
Educational reformers of the Victorian period believed that children needed long holidays to be with families and to get over the stress and pressure of education. Working class pupils had to work as well as attend school so they needed the extra holidays to help with finances at the most crucial emploability times.
Orignally I think most school terms revolved around the Church year ( and that also explains why the holidays are where they are, the terms are called what they are and why they are long. Churches were the first providers of education). The church calander, with its holy days and farming needs are most probable - so all hands - including those of land owners could get on deck and sow and plough and harvest and time was given to reflection and worship when the task was over.
I also recall as a lad having additional autemn half term holidays ( my current school retains this) for both potato picking and hop picking. It was an important source of income to working people and important to the farmers too who needed the labour.
My school ( state school) holidays were long. One of the reasons I have taken my DS out of school is because I think the holidays and the pressures are stupid and have nothing to do with education. When you are an older parent , and have lived a bit, you realise these things.