This is the 1615th most frequent Polish word.
marcu
"Marcu" is the locative or dative case form of "marzec," which means "March" (the month) in Polish.
The word 'marcu' is used here as the locative case form of 'marzec' (March), indicating the timeframe during which an event occurs.
W marcu zaczynamy nowy projekt.
In March, we start a new project.
Here, 'marcu' is again the locative case of 'marzec', used to specify the period of time being referred to.
Czy w marcu będziecie na wakacjach?
Will you be on vacation in March?
'Marcu' implies the month being discussed, functioning as the locative case form to specify when the described condition occurs.