Just a quick question

I'm moving condos in 2weeks , I have my contract already etc . Now I know you are supposed to inform immigration about this , my question is can I do it before I actually move , have to do it on the day , or is it ok to do 3-4 days after ?

When I last moved , I didn't inform them at all to begin with and my estate agent never explained and claimed they never knew it had too , so after being there 4 months I was told about it , went straight there next day , got a lecture for being a naughty boy .... I never asked and wasn't told when I needed to do if I moved again as at the time I didn't think I would be again ,any ideas ?