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2.
The season when a woman's breasts are made visible by wearing anything with a low-cut neckline. Usually in spring or summer when the temperature is higher than 75 degrees Fahrenheit. Also the time when young boys notice that girls in their seventh grade class have been growing breast during the winter.
1. Six more weeks till cleavage season.

2. It's cleavage season! Be careful when driving and be sure to keep you eyes on the road.

3. It's best to go to hard-on alley during cleavage season.
by SPIKYCLOUD June 25, 2009
 
1.
That time of the year after winter. Girls stop wearing heavy coats and start showing cleavage. But if its too cold and their nipples pop out, they go back under for another 6 weeks.
Tom: "Enjoying the view, friend?"
Tyler: "Oh yes, yes indeed. Im so excited its the beginning of cleavage season, I hope Sarah's nipples don't get hard."
Tom: "Yeah, it would suck to have to wait 6 weeks to see her flaunt her cleavage again."
Jordan: "I LOVE CLEAVAGE SEASON!!!"
by tits-o-plenty January 01, 2011