We have no idea where Julia is. Maybe she's chatting up a potential suitor in a parlor in an English palace. Maybe she's dancing a waltz with our adoring speaker. Hey, maybe she's strutting her stuff down the catwalk, smizing it up at the gawkers in the surrounding seats.
The where is not what's important. It's the what that counts here—what she's wearing, to be precise. In any case, we can imagine that this Julia woman is pretty well off, what with her silks and glittering gowns. And since Herrick was pretty well off himself, we can imagine he'd be familiar with the rich world of wealthy women in 17th-century England. Herrick, remember, was all about the lavish ladies.