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.
The theme of change could also be present. If the mistress is the 'last', there must be some kind of transformation or decision that leads to this finality. It could be a change in the protagonist's values, or external circumstances that force a break from this kind of relationship. For example, a change in financial situation, family pressure, or personal growth might all contribute to making her the last mistress. This theme of change can also be related to the idea of moving on from a life of secrecy and passion to a more stable or different kind of life.
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.
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.
Hope could also be a theme. Despite being in a dire situation and making what might be their last radio call, they still have hope that someone will hear them and come to their aid. This hope might be reflected in their voice, their words, and their determination to get their message across even as their situation worsens.
Another theme could be perseverance. The idea of sticking with something until the very end, through all difficulties, is a strong element. It might be about a character persevering through a long illness, a tough journey, or a difficult relationship.
One possible theme could be change. Since it's the last first time, something is about to shift or end.
Reflection might be a key theme. As it's the last shift based on a true story, the person involved is likely to reflect on their entire tenure at that job. They might think about their achievements, mistakes, and how they've grown. It could also have a theme of new beginnings, as the end of this shift is the start of something new, whether it's a different career path or a more relaxed life after retirement.