2. The length of Shiva:

Shiva begins right after the burial, and lasts for seven days thereafter. The day of burial is counted as the first day of Shiva even if the burial took place very late into the day. [Furthermore, even if the burial was completed during Bein Hashmashos, the previous day is counted as the first day of Shiva, as brought in Halacha 3. Nonetheless, there are cases of exception, explained throughout Halacha’s 4-12, in which the Shiva may end earlier or later than seven days from the burial. See there and in Chapter 27 for the full list of cases.]

Shabbos:[1] The day of Shabbos does not cease the mourning, and the seven days of Aveilus thus includes Shabbos.[2] Furthermore, in certain cases, Shabbos can even count as the first day of Shiva, as explained in the Halacha 3!

The last day of Aveilus: See Chapter 21 Halacha 1 regarding the exact time that the Shiva ends on the seventh day.

Yom Tov breaks Shiva: The arrival of a Yom Tov ends the Shiva as explained in Halacha 11 and Chapter 27 Halacha 1. See there for the full details of this matter!

Shiva calculation chart

*Excluding Yom Tov/Chol Hamoed, and other cases of exception explained in chapter 13 and 27 in which the Shiva may begin some time before or after the burial!

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[1] Michaber 400:1; Mishneh Moed Katan 19a

[2] The reason: As some of the laws of mourning are applicable also on Shabbos, such as private mourning laws. [Michaber ibid]