It's the first day of May and Corinna's in bed, probably sleeping off a long night of A.P. Bio cramming. It's not clear whether the speaker is inside her bedroom or standing outside her window, but either way, he's using all his poetic might to wake her up. That means that although the poem celebrates spring and nature and everything wonderful outside, the physical setting of the lovers is inside. It's an indoors poem about the great outdoors.
And what about this outside? A Google Maps search would locate Corinna in some rural English village—somewhere small enough that the whole community is celebrating this festival, where "each house" is decorated with flowers (32) and every "budding boy or girl" (43) is out courting and eating cake.
We also know that this village is no city suburb stuffed with Wal-Marts, Chick-fil-As, and miles of well-lit parking lots. This place merges almost seamlessly with the surrounding forest:
Come, my Corinna, come; and, coming, mark
How each field turns a street, each street a park (29-30)
Plus, the festivities Herrick describes here are all rustic traditions, part of the English countryside's commitment to old-time cultural fun.