TL;DR: No need to hoard the presents; it won't do anything.
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Santa presents are now tied to the day they were given, not the day they are
opened.
This means that you won't benefit from opening them en masse on Christmas eve.
You can do it for fun if you so prefer, though.
Only the present given during the boot starting on 24th day, the last day
Santa is around, is special.
As a side effect, you probably should be able to open the presents even on
25th, and their contents wouldn't be any different. Or 26th, if you so fancy.
Thank you to the mysterious wizards and archwizards who had the time to fix
this.