It's called "Lupercallia", and it's by Julian Lee.
Set during the days of the Marauders, it follows Sirius around as he follows Remus around trying to figure out why Remus keeps talking to Snape.
Until James informs him that the day after Valentine's Day is the day that wolves choose mates. Wolves (and in this case, werewolves) mate for life. Sirius sees read at the idea of Snape having Remus. And as Lupercallia comes nearer, Sirius tries to keep Remus away from Snape. Snape isn't good enough. He doesn't understand Remus. Snape shouldn't be the person Remus is with for the rest of his life.
Warm squishy ending with Remus getting Sirius, and plenty of young Snape snark to go around.
Webpage is here:
That's the main page, and you can easily navigate from there.