For most boxes, the content is determined when you first open it, and it's likely (but not certain) that 'next day' happens at the daily quest reset time, 6 am edt for pc/NA for example.
For these particular boxes, though, most people will probably be getting lots of boxes every single day of the event, so holding onto boxes is probably not sustainable or beneficial. If it's the last day or so, you're still missing bonemold pages and want to hold boxes to open over the next few days that might make sense, or if your situation is such that you can't play throughout the event.

No point holding on to the boxes in my opinion, you can get them so easily.
I'll be opening 105 boxes each day of the event just from doing daily crafting writs lol

jrfk2 wrote: »

So the write up for this Anniversary event says:

"New: A small chance for a tradeable outfit style page for the Bonemold armor style
If you’ve already collected a Bonemold armor style earlier that day, you will instead be rewarded with either a Saberkeel style page or a Worm Cult style page"

So once I open an Anniversary Jubilee Gift Box and I get a new armor style, should I hold off opening any more boxes that "day" in the hopes one of them may give me another piece the next "day"?
Or are the gift box contents predetermined when it is awarded and not when you open it?

If indeed the contents are determined when you open it, what is a next "day"?

For this event we might not get an answer, since people are getting multiple Bonemold pages per day.

Got 4 today. We'll see if they fix this before the end of the event.