Okay do they have saints in Thedas?
Short answer: Probably not.
Long answer: In my defense, Andrastianism is Fantasy Medieval Christianity. The many spirits of Andraste's allies you encounter in the Temple seem like expies of Jesus' disciples, and the Chantry itself seems to have a real persecution complex (and they're quick to turn Andrastian humans who died for their beliefs into martyrs), so I imagine they could conceivably have the Thedas equivalent of saints... if a fanfiction writer so chose to write about a made up one... 
Feast of St. Valentine - Valentine Amatius, Tevinter priest who helped shepherd rebel lovers to escape the Evil Empire; he eventually traveled south and became a brother in the Orlesian Chantry.
EDIT: Okay I'm planning a (short) fic response to the Sera prompt and maybe a few more 100 word flash fics!
That sounds good, although you gotta slip that he had a lover in there somewhere! 
I know little about the story of the actual (probably fictional, let's be honest) Saint Valentine. I heard a colloquial story that he was he got locked up by the Roman Empire (because they were persecuting Christians at the time, of course), then right before he was executed he sent a letter to a woman he was really close with (I can't remember if they were in love or engaged or just had feelings for each other from a distance) basically comforting her, reassuring her that God would reward his faithfulness with heaven, then signing the letter, "From your Valentine."
Because of this, this canonized saint that some medieval scholars wrote hagiography about (which is almost certainly not historically accurate; in fact, it's doubtful the guy ever existed), then became associated with courtly love. Which eventually became just regular romantic and sexual love, because people. 
Anyway, throw in a reference to this Thedas Valentine having a lady love (perhaps one he sent a sweet letter right before getting the chop) and we're in business.
Eeexcellent! The "do what you want"s prevail! As always, honestly, do what makes you happy. The prompts and ideas are there to help. Just make it culleny. 
I sort of wrote a story for my Sera prompt with Siri while driving to work this morning. We'll see if I get it finished up into a proper story! It was pretty hilarious, tho, hearing Siri read it back to me. "Cullen heads up the stairs, but that guy whose name I don't remember tries to stop him" LOL
Did we come up with anything for replacing Vday with something else? I poked around and there doesn't seem to be anything in canon. There are a couple of Thedosian holidays that celebrate marriage, but they didn't sound the same (more akin to May Day, etc). There are feastdays, but THE "feastday" is April Fool's Day.
For myself, I think I may just go with "Valentine's Day." Modern Vday isn't about the historical significant of St Valentine anyway, just as Thedas didn't culturally develop English or "Tuesday." So I'm going to squint... and co-opt curiousartemis's idea:
Yeah... that's the downside of Thedas being a shallow, surface-level fantasy counterpart to real medieval kingdoms, cultures, religions, and holidays. It's useful in some ways, but if you wanna dig deeper, you'll hit rock bottom in two inches.
In my digging since this morning, I realized that "feastday" is just an archaic word for "holiday." Granted, the Medieval Catholics had a million of them because they have a million disciples and saints and martyrs to celebrate (there's basically a saint for every day of the calendar), but, you know... some were more popular and had bigger celebrations than others. (You could definitely see where this one would appeal to the masses.
)
"Modern day Thedosians celebrate the Feast of St. Valentine often without knowing its deeper historical roots in Tevinter. While today it consists of ostentatious demonstrations of affection between lovers, the feast commemorates Valentine Amatius, a priest known to help slaves escape from Tevinter. His tireless efforts to end slavery were overshadowed by romanticized tales of shepherding rogue lovers from the strictures of Tevinter society. When he later escaped south himself and joined the Orlesian chantry, the bards of Val Royeaux immediately popularized his tale. Now he is considered the patron saint of lovers, and once a year, his feastday is recognized in Orlais, Ferelden, and parts of the Free Marches with the exchange of too-expensive gifts and chocolate. The more lavish celebrations wane, but the chocolate remains."
--Brother Genetivi, Holidays, Celebrations and Ritual Bloodlettings in Modern Thedas

I LOVE THIS!
In my fic, I'm totally gonna have Chantry clerics lament that this is supposed to be a serious holiday celebrating the religious devotion of a devout Andrastian who did good works for early, pre-Chantry Andrastians... yet to everyone else this holiday is just an excuse to throw feasts, festivals, and smooch a loved one.
*GASP* Maybe we could approach this Hunchback of Notre Dame style?
In the first movie, there was the Festival of Fools, obviously an expy of April Fool's, which the Feastday DLC of DAO is clearly meant to represent.
In the second movie (which honestly hurts my soul because of how bad it is x_x), there was the Love Festival, obviously an expy of Valentine's Day, which we can use for writing inspiration if we so choose. 