One possible theme could be the idea of seizing the moment. Since it's just a weekend, the characters might realize that time is short and they need to make the most of their time together, which leads to the development of love.
It could be a story full of sweet and tender moments. Maybe it's about two people who met during the last weekend and instantly felt a connection. They might have gone on a long walk on the beach, shared their dreams under the starry sky, and as the weekend ended, realized they were falling in love.
There are several possible themes. Escape could be one. People might be escaping from work stress or personal problems during their weekend away. Another theme could be cultural exploration. If they travel to a different place for the weekend, they could be immersed in a new culture, learning about different traditions, languages, and ways of life. Also, the theme of renewal is possible. A weekend away can give people a chance to recharge their batteries, both physically and mentally, and come back with a new perspective on life.
Love and loss could be a theme. Often in love novels, the characters experience the joy of falling in love and then the pain of losing that love in some way.
One possible theme could be love and lust. Since it involves a mistress, there's likely to be an exploration of the passionate and often complex relationship between two people who are not in a traditional marriage or committed relationship.
One possible theme could be change. Since it's the 'last' Christmas, there might be a change in the characters' lives like leaving Vermont.
Another theme might be change. If it's the last day of an era, it shows how things are about to transform. For example, a society that has been stagnant for a long time may be on the verge of a big shift after this last day, and the story could explore the resistance or acceptance of that change.
Love might be a theme. Maybe there's a love story that reaches a turning point on the last day of June. It could be a day of rekindling an old love or saying goodbye to a loved one.
Another theme could be truth. Maybe the last answer is the truth that has been hidden throughout the story. It could be a moment of realization for the characters or the readers. For example, in a mystery short story, the last answer might be the true identity of a mysterious figure. Also, redemption could be a theme. The last answer might be a way for a character to redeem themselves, to make up for past mistakes or wrongdoings. It could be a powerful moment of self - discovery and growth for that character.
Love and loss could be a theme. The last contact might signify the end of a relationship, and the story could explore the emotions that come with that. For example, regret, longing, or acceptance.
Perhaps there were some exciting outdoor activities like mountain biking or kayaking.