"'#333: Underneath a large, egglike rock east of Vanhoover'," Twilight read aloud. She lowered the centuries-old parchment, examining the matching boulder before her. "Well, this looks like the place."
Her horn ignited, and the stone slid aside. Long ago, this maneuver would've been grueling, even for the young alicorn she'd then been; nowadays, not so much.
The Princess of Friendship instantly recognized the relic occupying the newly exposed crater; after all, she'd uncovered many identical others over the years. With eager anticipation, alongside an equal desire to savor the annual discovery, Twilight opened the box.
As always, five multicolored packages lay inside, but — also as always — Twilight seized the letters attached to each first. She picked one at random, then levitated it up to her muzzle.
A tear trickled past her wistful smile as she read her old friend's faded hoofwriting:
Happy three hundred and thirty-third birthday, Twilight!!!
Pinkie always plans in advance, but to also plan to include the others in the plan so Twilight would get gifts and letters not only from Pinkie but from all five of her friends centuries later? Pure genius that is Pinkie.
...Of course, one wonders just how many ways five separate ponies can say "Happy Birthday" several hundred (possibly thousands of) times before running out of ways to say it, especially when they're not there to keep track of current events. Then again, Twilight would undoubtedly be happy with just that: a Happy Birthday from each one with their own signatures, even if it repeats hundreds/thousands of times, she'll value each one as the unique gesture of love and remembrance that they were intended to be.
Kudos for this wonderful idea!