This is the 616th most frequent Hungarian word.
nyári
"Nyári" means "summer" or "summertime" in English.
The word 'nyári' functions as an adjective here, describing the 'szünet' (vacation) as something related to the summer.
A nyári szünet júniusban kezdődik.
The summer vacation starts in June.
In this sentence, 'nyári' acts as an adjective modifying 'napokon' (days), specifying them as pertaining to summer.
Nyári napokon szeretek kirándulni.
On summer days, I like to go hiking.
Here, 'nyári' serves as an adjective describing the 'ruhám' (dress) as suitable or intended for the summer season.