This poem is divided into three six-line stanzas, which gives it every appearance of regularity. In fact, if you only counted the first two lines of each stanza, you'd even say that it has an even number of syllables in each line. (After all, lines one and two each have twelve syllables.) Come to think of it, the first two lines are chock-full of trochees. That's a fancy term for pairs of syllables which couple an accented and an unaccented syllable together to make a DUM-da sound. Here's what we mean. If you read the first line, it would sound something like this:
See? There's a regular pattern of accented and unaccented syllables.
Here's where things get interesting, though: after the first two lines, there's not any regular metrical pattern. Why would Millay set up such a regular system and then forget about it completely?
Well, here's our best guess: see, the speaker wants to remember this night more than anything. In fact, the most important part of the poem is the fact that she repeats her impression of the night over and over. Making the first lines fit into a regular metrical pattern makes them easier to remember….and after that, she's not so worried about the actual details of the night. That's why the rest of the lines fall into no particular order. It's not what they do that matters – it's the fact that she can remember it that counts.