Herbs such as mint should be harvested in the morning, on a sunny day after all moisture has dried from their leaves. It's important to harvest before noon because moisture leaves the plant later in the day and decreases its aromatic properties.
Harvest before the mint flowers open. A spike of tiny blooms will begin to appear on the tops of your mint plants around the middle of the growing season. Texas A&M University's website advises you to harvest your mint leaves before these itsy bitsy blossoms begin to bloom.
Mint is a perennial herb, and the University of Vermont website notes that perennial herbs should be harvested in no later than a month before fall frost.