Well . . . that is a challenge. I hope I am not too late for your needs. You could try . . . precursoversary. Precursor is something that comes before, or in some definitions an advanced message of something to come. So a percursoversary would be the turning back to something that is to come. Anniversary is composed of yearly and turning, so returning to something yearly. So a precursoversary would be turning to something that is to come or that comes before. We get the word verse from the same root, where the line turns in a poem. You could play with the spelling. I couldn't figure a way to say yearly without making the word too cumbersome to say easily. I hope that might at least give you some idea how it might be done. You project sounds fascinating! Good luck!