In the Jewish mourning tradition, mourners begin sitting shiva immediately after the burial of the deceased. Shiva lasts 7 days as the word shiva means 7 in Hebrew. The first day of shiva is the day of the funeral, so even though it’s a half day it counts as the first day.
There are a number of other rules that can change the amount of time that you’re supposed to sit Shiva for. If a Jewish holiday falls during the period of time you’re sitting shiva, shiva ends the afternoon before the holiday. For example, if Yom Kippur falls during the 7 days of shiva, shiva ends the afternoon just before Yom Kippur begins. Even though the holiday decreased the time spent sitting shiva, it is considered that you mourned for seven days. In the case that a person passes away during a holiday, the burial and shiva should wait until the holiday is finished. There are also some rules about when to sit shiva if someone passes away on Shabbat. In this case, the burial is done the next day and then the shiva period begins that day after the burial.
If you’re already sitting shiva when Shabbat arrives, you should remove the outer signs of mourning such as covering the mirrors, wearing no makeup, sitting low, wearing torn ribbons/KaddishBands etc. This is because Shabbat overrides sitting shiva. However Shabbat does count as one of the 7 days of sitting shiva even though you’re not focusing on sitting shiva. When Shabbat is complete, that Saturday night you begin sitting shiva again. You can cover the mirrors again, etc.
The purpose of sitting shiva is to allow you time as the mourner to focus on your grief and have your family and friends there to support you. By covering the mirrors, not showering or putting on makeup, wearing a torn ribbon or KaddishBand, etc you are allowing yourself to focus on the soul and not worry about keeping up the physical. Sometimes in the more modern times sitting shiva turns almost into a party type atmosphere as many friends and family you haven’t seen in a long time come to your house.
Just remember that this is your time to mourn and you should do whatever makes you comfortable. It’s important to take this time, even if you think you don’t need it, before returning to your daily life.