This is the 97th most frequent Finnish word.
maanantaina
"On Monday"
Here, 'maanantaina' functions as an adverbial expression of time, specifying the day of the week an event will occur.
Tapaamme ystäviä maanantaina.
We will meet friends on Monday.
In this sentence, 'maanantaina' specifies the day on which the described weather event occurred.
Maanantaina satoi vettä koko päivän.
It rained all day on Monday.
Here, 'maanantaina' indicates the routine day of the week when the subject goes to the gym.