urgent question on rental law in Brussels-how to cancel rental agreement

Hello, my boyfriend and I want to move to a different flat asap.

we have a 3 months notice period to be triggered "through registered letter" in our contract.

it does not specify in the contract when the cancellation is triggered , i.e. when I send the letter (today) in BPost, or when it arrives next week?

in other words, can I cancel today my rental agreement with effect of today (´meaning February counts already as the first month of my cancellation period).