Antonym/Opposite of "on the morrow"

If my birthday is on the 15th August and I tidy up on August 16th I can say: "I threw a huge party and tidied up on the morrow.". But if I prepared for the party on August 12th then what do I say? "I threw a huge party which I prepared on the ......". I don't want to use "the day before". Is there another more eloquent word/phrase? Some thoughts:on the yesterday, yestern, ereyesterday.

Solution 1:

Since nobody has yet included it in an answer:

The most obvious parallel to on the morrow (which is archaic and highly literary and not something I would recommend using in normal speech except if you’re deliberately trying to sound like someone out of a Tolkien or Shakespeare piece) is on the eve.

Eve is slightly less archaic than morrow, and it is still relatively common to hear the phrase on the eve of X used in normal speech; so you could say that you held the party on the eve of your birthday (you had the party the night before your actual birthday).

Without the prepositional phrase of X, however, it becomes very archaic and stilted, just like cleaning up on the morrow is.