rainy season vs time of year

rainy season and time of year both are nouns.

Word	Noun	Adjective	Verb	Adverb
rainy season	Yes	No	No	No
time of year	Yes	No	No	No

As nouns, time of year is a hypernym of rainy season; that is, time of year is a word with a broader meaning than rainy season:

rainy season: one of the two seasons in tropical climates, a yearly period during which most of a country's rainfall occurs
time of year: one of the natural periods into which the year is divided by the equinoxes and solstices or atmospheric conditions

Other hypernyms of rainy season include season.

rainy season (noun)	time of year (noun)
one of the two seasons in tropical climates, a yearly period during which most of a country's rainfall occurs	one of the natural periods into which the year is divided by the equinoxes and solstices or atmospheric conditions