How does mensuration occur? where does it occurs

I think you meant MensTRAtion.

If you did, it occurs after the hormone levels drop at the end of the woman's previous cycle where it would be signaling blood and tissue lining in the uterus to break down and shead from the body. It could last to about a week. It occurs in the uterus and goes through a 28 day cycle to your next period.

If you did not mean menstration I am sorry.

Mensurations, if that's what you actually meant, is basically the theory of measurement. It occurs in geometry. It measures geometric magnitude, lengths, areas, and volumes.