This is the 675th most frequent Estonian word.
märtsil
"On March" (referring to time in the month of March).
In this sentence, "märtsil" indicates that the event occurs within the month of March.
Tal on sünnipäev märtsil.
He has his birthday in March.
Here, "märtsil" specifies an exact day within the month of March.
Üritus toimub teisel märtsil.
The event takes place on the second of March.
In this usage, "märtsil" generally refers to events happening during the month of March.