If the blu-tac falls during the night, how would we know when or which day it fell?

I suggest that we choose a heavier weight from the science lab and attatch it to the blu-tac,ensuring that it would drop within a day,and thus making it more convenient and accurate for us to measure the time taken for the blu-tac to fall off the wall .

But if it is like this, then wouldn't we need a stopwatch instead of a calender to calculate the time taken?