Talk:Princess Celestia/@comment-7345250-20141123093614/@comment-24908786-20141123180125

The Journal of the Two Sisters also mentions that it was Star Swirl who asked Celestia and Luna to become Equestria's sovereigns, and that was before they got their cutie marks.

Then, they helped him regain his power after he exhausted it, gained their cutie marks, and continued to help Star Swirl sometime afterwards, so Celestia's meeting with the Parallel Sombra happened sometime around the 1,000 years before the series started.