This is the 638th most frequent Finnish word.
heinäkuussa
"In July."
'Heinäkuussa' acts as a temporal adverb indicating when the event occurs (July).
Heinäkuussa aurinko paistaa usein kirkkaasti.
In July, the sun often shines brightly.
'Heinäkuussa' here specifies the occurrence of the verb 'käymme' ('we go') during the month of July every year.
Käymme lomalla heinäkuussa joka vuosi.
We go on vacation in July every year.
'Heinäkuussa' sets the time frame for the summer events being organized, referring to July.