History:

The Aubade, which as a word means “dawn serenade”, first dates back to the 12th century and came to be known to English speakers in the 1670s. It is theorized by some scholars that it grew out of the cry of the medieval watchman announcing the change from night to day from his tower. It is a lyric poem that celebrates the dawn, and is often a love poem about lovers parting at dawn. Popular and well known Aubades include by Phillip Larkin, John Donne and Chaucer. The Provençal, Spanish, and German equivalents are alba, albada, and Tagelied.

Structure:

Although it is traditionally written in iambic pentameter, the Aubade does not strictly have a formal structure, is left up to the poet, and is defined more by it’s subject matter and tone, that being of longing, ecstasy, love, and heartbreak at having to part.

So there are no structural rules, traditionally and strictly speaking, that you have to follow, and throughout history poets that write Aubades choose structural rules themselves. Larkin’s Aubade, for example, is elaborately structured, by his own choosing.

Sources:

https://www.shmoop.com/aubade-philip-larkin/rhyme-form-meter.html
https://www.poetryfoundation.org/learn/glossary-terms/aubade
https://en.wikipedia.org/wiki/Aubade
https://www.washingtonpost.com/archive/entertainment/books/2003/03/16/the-aubade-is-a-dawn-song-expr/45ddb7b7-623f-4b76-90b2-59d06f40e5e0/?noredirect=on
https://www.enotes.com/topics/aubade-philip-larkin/in-depth
http://www.howtodothings.com/hobbies/how-to-write-an-aubade