This is the 248th most frequent Czech word.
sezoně
"Sezoně" means "season" in the dative or locative case in Czech.
'Sezoně' (season) is used here in locative case to specify the time period during which an action occurs, 'in the summer season'.
V letní sezoně navštěvujeme mnoho festivalů.
In the summer season, we attend many festivals.
'Sezoně' is again in the locative case, indicating a specific time frame being discussed, 'in this season'.
Máme nové zvyky v této sezoně.
We have new habits in this season.
'Sezony' (season) is in genitive case here to express 'during the season', showing possession or association.