I'm only going to cover the characters who are in a canon relationship from my books. It won't be conclusive and I cannot write official chapters because it will go against the contract clause to publish elsewhere. However, I can give you an idea of what might happen on this lovely day :D
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Laura & Tynan (E-Ghost)
Seeing how Laura is an extrovert and Tynan is an introvert, they would have Valentine's Day as a couple indoors, spending quality time together. Tynan is a busy business owner and Laura is a professional dirt bike racer who is often travelling. I assume that Tynan would book them a proper getaway to some place undisturbed. Although the couple is married, they are still very much in their honeymoon phase.
Scarlet & Edward (Love Journal)
This is a young couple who had been through a lot in life. I suppose Valentine's Day is special for them as students. Something like a movie date or a simple day out with each other at amusement parks alone would be nice. On the other hand, there is a possibility that Scarlet will spend her day with her family and Edward will spend his time with them. That includes her sister Alex and her sister's boyfriend Jasper.
Emily & Richard (Single Dad)
Here, we have Emily the new graduate and Richard her CEO husband with a furry secret. Due to their pre-started family, it's not going to be a two-person Valentine's Day. Although Valentine's Day is meant to inspire the romantic vibes, this couple prefers to spend it more practically catching up on sleep together, exhausted with raising children.
I'm not a very romantic person, unfortunately. This is probably why the only good romances I write exist in BL fan fictions LMAO