Owning your own home

As above - 'at' if that's the language used, means on the day to me.

I would even argue that means when the job is finished (same day).

@Jameo I'd say the same.

Seems like an odd way to phase it too. Why not tie it to a specific date?

Ultimately though you've signed the contract, so you don't really have a justification to go back to them now with criteria on holding back a %. You can try, but they can just say no.

I would say something along the lines of our contract says "at", not before or prior. So I'll pay on the day as per the contract. I've been really happy with the process so far and chose you for the quality of your windows and reputation.

Then if they press it some more then tell them it's their contract and if they're going to cancel the job you'll need your deposit back and compensation for the time you taken off for the day of the install. They're breaking the contract with such little notice that you won't be able to get work for that day. Then something about how your looking forward to the windows and this isn't inline with your experience to date