Jabed, whatever you want to call it and however you want to look at it, children are not going to make progress over the 6 weeks and are likely to forget stuff/lose skills.
Make progress at what precisely? I think you have a very narrow definition of progress there. Holidays are a time when children make progress in other things than school based ones. Even if they are not making progress, what does it matter? Its a holiday. We all need time out. Children more than anyone else.
It takes a long time to get students back to the point they were at before the holidays in September.
I ask again what is it you are doing ( or not doing) differently to me and other teachers I work with? I have not found it taking any time at all for students to get to some point they were supposedly at before the holidays.
I know precisely where they were before the holidays and when we come into school refreshed after our 8 weeks summer break we all hit the ground running within a couple of days and get on with what we are doing. There is no so called regression or slow start or having to get back anywhere.
TBH it doesn't need a whole lot of research to prove the obvious. Learning maths is not much different from playing the piano - it takes practise.
I agree about the practise but I do not agree that you need constant practice. There is always time for a break. Often it is needed. My DW is an accomplished musician - concert level. She often takes breaks but whilst it may have left her rusty for a couple of days it didnt have a serious effect. Piano and maths and riding a bike , like anything else are skills once learned are not forgotten and it doesnt take long to come up to speed again.
I repeat, if the students are regressing, then the task has not been learned.