Yep! I don't think it's relevant to go into details of what it's supposed to do unless there is no way to remove the week 53. Just to give a quick story short, I'm taking into considering last week's data, the 4 last week's data as well as the last 13 week's data based on a database being updated weekly.

Also, it would be possible to resolve this with an IF formula, but it would take a long time to do and apply on every single formula I already have...

If (weeknum(XYZ)=53,1,weeknum(XYZ))

Also, even if I do that, it might be problematic when I'll be in 2015...

Additional details :
Thee weeks are from sunday to saturday. So basically, week 1 would include sunday 29th 2013 to saturday 4th 2014.

Instead of having sunday 29th, monday 30th and tuesday 31st as week 53.