This is the 597th most frequent Hungarian word.
hétvégén
"On the weekend."
Here, 'hétvégén' specifies the time when the action (having a picnic) will take place.
Hétvégén piknikezni fogunk a parkban.
At the weekend, we are going to have a picnic in the park.
In this sentence, 'hétvégén' indicates a recurring time frame, specifically every weekend.
Minden hétvégén látogatom a nagymamámat.
Every weekend, I visit my grandmother.
Here, 'hétvégén' refers to a general time frame, indicating the weekend, during which the speaker is unavailable.