Well, without having read 'ayakashi oyado ni yomeiri shimasu light novel', it's hard to give a detailed account. However, light novels often have unique storylines. The 'ayakashi' part makes me think it could be a supernatural story. Maybe the main character goes to an inn ('oyado') and gets involved in some sort of strange events related to ayakashi, and the 'yomeiri shimasu' might be a key part of the plot, perhaps symbolizing a new start or a special bond within the story.
I'm not sure specifically what the'sama sama letter day story' is about as it's not a widely known common story. It could be a very personal or a story from a specific local or cultural context.
I'm not privy to the main characters in 'ayakashi oyado ni yomeiri shimasu light novel' since I haven't read it. However, based on the title, it could be a young person who, for some reason, has to stay at the 'oyado' where the ayakashi are. Maybe there's a female lead who is trying to understand and deal with the ayakashi she meets there, and her experiences could form the crux of the story. Or it could be a male lead with some special abilities that make his encounters with the ayakashi and his stay at the inn very interesting.
The Kaguya-sama manga is indeed finished. It ended after a successful run, leaving fans with memories of an engaging and enjoyable story. The final chapters tied up loose ends and delivered a conclusive ending.
Yes. The Kaguya-sama manga has reached its conclusion. It provided a satisfying ending for fans. The creator wrapped up the plotlines and characters' arcs neatly.