Wiktionary
n. The first rainfall of a season, at the start of spring in cold climates or the end of summer in hot, dry climates.
Wikipedia
First Rain is an annual event held at the University of California, Santa Cruz (UCSC) which began at Porter College. It is also known as the Naked Run. During the first rainfall of the school year, students run naked through the campus, generally starting at Porter College.
Usage examples of "first rain".
I added a gray shirt that looked as though it would hold its shape as long as nobody washed it, a tie with a picture of a Balinese maiden, a pair of ventilated shoes with steel taps on the heels that would be all that was left after the first rain.
And now, on this, the first rain- and snow-free day for ages, that oleaginous lardball admiral had to accost her.
At first, she heard only the wind, and assumed the water, as the canopy absorbed the first rain.
Joseph watched until the first rain began to fall, and then he sauntered to the porch and sat on the top step, in front of Elizabeth.