Let's suppose you live near a place called the Rose Garden, which is full of (surprise) flowers. You and your best friend spend a lot of time there, especially when you're bored and don't feel like doing anything else.
One day in June, shortly after graduation, you start talking about how you really want to spend some time in Europe. You're worried about leaving home, though, and you can think of all sorts of reasons not to go.
You weigh the pros and cons with your friend, who, perceiving an apt metaphor in the roses (he remembers Herrick's poem from an English class) tells you to "gather ye rosebuds while ye may." Huh? He explains that this opportunity isn't going to be around forever. Pretty soon you'll have a family, a job, and then it will be too late. You have to pick the roses while they're fresh, ripe, available. The combination of your friend's wisdom and the peculiar colors of the late afternoon sky prove quite convincing.