The main idea of this poem is welcoming the new year and spring as an opportunity for a fresh start and new love. The closing lines sum up the main idea well:
"Then you faire flowre, in whome fresh youth doth raine,
prepare your selfe new love to entertaine."
The 'fair flower' he is referring to is a woman, and he is telling her to get ready ('prepare your selfe') to entertain (discover) new love!
