I suddenly find myself trying to describe a date that's an exact number of years before a scheduled event and I can't think of a better word to describe it than "pre-anniversary" or maybe even "preanniversary". Both seem rather ugly and open to misinterpretation, for example I can see how they might be interpreted as meaning some arbitrary period or point in time before a specific anniversary date.
So how might I be more specific?
Example: something is known to be going to happen on, say, 20-May-2024. I'm looking for a word to describe 20-May-2023, being exactly one year before (the "minus-oneth" anniversary). Or other 20th Mays even further back.