The Stardew Valley calendar can be accessed either outside of Pierre's General Store or by purchasing a calendar from Robin at the Carpenter's Shop for 2,000g. You can hang this on the wall of your house, which makes viewing the calendar much more convenient.
The calendar tells you the days of the villager's birthdays and when there are special festivals. Birthdays are important because gifts given on someone's birthday will benefit from a large boost in friendship points. Festivals are fun events that are optional but recommended to attend. You will receive a letter from Mayer Lewis before each festival reminding you of it. On festival days, you do not need to feed animals.