Honestly, it was there mostly because if felt like something had to be.
When she was younger, the calendar was filled with all sorts to things: when the nicer grocery man would drop off deliveries (he was a kind young man with a pretty face and plenty of candy to share), the days her parents didn't need her at the restaurant, and Jirochou's birthday.
Her parents never paid it much mind, only jotting down notes for big things that didn't need reminding: birthdays, the few holidays they celebrated, and anniversaries. They often forgot to change the month or date, so she would do it instead.
As she grew, the calendar was no longer full of fun little events; she no longer had the excitement and love of life her younger self did. Perhaps it was due to the passing of her parents, or perhaps it was the growing distance between her and her best friend.
She filled her days with work, noting her schedule and memorials on the calendar. One day, she would buy back the building that once was her family's restaurant and home. One day...
After she married, the calendar returned to it's previous home: it hung on the wall, the same spot as in her childhood. They filled it with plans for renovations, their work schedules, and anniversaries. Sometimes he would write words of encouragement in the corner.
It was filled with marks of their life and love. Such a small thing, but it felt so special. She loved it dearly.
When he died...Why even bother counting the days anymore? She didn't throw it away. She just stopped caring.
There were no more schedules, no more words of encouragement. There were only days that passed in a blur yet felt stretched thin, lasting forever. There was no point in tracking the days, it would only remind her of what she lost, what she would never have again.
Heartbroken, she drifted through life without truly living for years. Before she knew it, her hair had grown darker and her skin started to wrinkle. People knew her more as Otose than her real name, and she had amassed a beautiful, idiotic, and loving family.
She started to smile and banter again. She rented out the upper floor of her building to a man who would later become the son she never had. And soon he would bring with him a family of his own: a glasses wearing otaku and an Amanto girl who ate her weight in food each meal. He became the most beloved man in Edo, all while upholding his promise to her husband's grave to protect her until the end.
The calendar had meaning then. There was so much to write that it barely fit: Gintoki's, Kagura's, Catherine's, and Shinpachi's birthdays, "rent" days (she was never actually serious about collecting it), when Taka needed maintenance, things to teach Catherine to do when she took over, and so many others!
She didn't need it for tracking the days; she was no longer dead inside. She had something to look forward to every day and plenty of love in her life. She was happy again.
It was no longer just a calendar to her. It was a reminder of all the beautiful things in her life, what she had to look forward to every day, every month, every year. It was a treasure to her.