After that suggestion, keeping in mind that all I 'know,' about Japanese holidays comes from wikipedia, TVTropes, and half-remembered fanfic....Parinirvana Day? Not traditionally romantic, according to the TVTropes description, but here's what I was thinking when I read the description: It's a day where you reflect on both your death and the recently deceased. Post UBW, I would seriously expect Shirou to reflect on both Archer and his own path, and Rin to reflect on Archer as well. You could either play it straight, with Rin and Shirou coming together after their reflections; or given how you've had an unusual post-death appearence by Saber at least once in a romance story, you could have Archer either show up independantly, or temporarily manifest in Shirou, and then have an encounter with Rin, a sort of resolution/last date thing.