I'm guessing it means 'from that day' rather than 'on that day'. Literally "with that day as a watershed". But that really is just a logical guess. I've never seen it before. If you're not familiar with the AをBに construction then try this link: https://japanese.stackexchange.com/questions/20854/meaning-and-transitivity-of-%e3%82%b4%e3%83%9f%e8%a2%8b%e3%82%92%e6%89%8b%e3%81%ab%e7%ab%8b%e3%81%a1%e4%b8%8a%e3%81%8c%e3%82%8b
– user3856370Apr 21 '19 at 16:58
1 Answers1
4
「その日を境{さかい}に」
「境」 in this phrase means a "turning point" where something changed.
What it is that changed must be explained (or at least implied) in the context before this phrase appears.