This is the 515th most frequent Czech word.
víkendu
"Víkendu" means "of the weekend" in English.
Here, 'víkendu' is used in locative case, accompanied by preposition 'o' which means 'over' or 'during'. The phrase specifies when the event is happening.
O víkendu pojedu na výlet.
Over the weekend, I will go on a trip.
In this sentence, 'víkend' is in nominative case as part of the temporal expression 'last weekend'. It specifies the time frame of the event being described.
Minulý víkend jsem byl na horách.
Last weekend, I was in the mountains.
Here, 'víkendu' is used in genitive case, describing the type of weekend that provides the direct object in the sentence.