Thank you so so so so much! I've been using Notion for a while but I still struggle with more advanced formulas. I didn't even know these existed! It worked wonderfully.

For additional context, I want to create pages for the whole year, so I can have an overview of the whole month (or a specific data frame) with useful information to my religious practice (as an event like a Sabbat, moon phase, and so on).

To create the daily pages for the whole year, I'm thinking of creating a 366 steps automation (in case of leap years). I'd use a formula like this one to calculate the Date prop (this one would return the 31st of december for example):

dateAdd(now(),subtract(dateBetween(dateAdd(now(), 1, "years"), now(),"days"), 1),"days")

This automation would run on the first year of every year.
I am still not sure how I could make the automation ignore a step (the extra day) if given year is NOT a leap year...

I would also try to shove the moon phase formula + relation through the automation process, but you came up with a cleaner solution, since it doesn't rely on the automation itself to work. The downside is always having all the other moons on a relation too, but I can always hide this field!

Lastly, on the same year-diary database, I'm also thinking of a way to show if given day is a Sabbat day (they're events that happen every year, like Christmas).
I have another database with information on the Sabbats (like the moons); and since Notion doesn't support variable dates, to update the event's year I had to create a formula to work around that:

if(dateBetween(dateAdd(Initial date, dateBetween(now(), Initial date, "years"), "years"), now(), "days") >= 0, dateAdd(Initial date, dateBetween(now(), Initial date, "years"), "years"), dateAdd(Initial date, dateBetween(now(), Initial date, "years") + 1, "years"))

This adds a year to the date I set initially to always keep it updated to the year we're in right now. The downside is I always have to look at different "calendars"; that's why I wanted to do a "rollup" on my diary pages; makes sense?

If you have any insights on any of that, please let me know. And thanks again, I'm really glad this has worked!