This year, (5781 / 2021) Tzav is a special Shabbat.
It is Shabbat HaGadol: (Lit. 'the Great Shabbat');
The Shabbat preceding Pesach / Passover is called Shabbat HaGadol because it was the day when the Jews were to take the sheep (which the Egyptians worshipped) to be used for the Korban Pesach (Pascal offering) four days later. (This means that the first Pesach was on a Wednesday).
Pesach / Passover starts Sundown Shabbat, March 27, 2021.
After nine plagues, the Egyptians were powerless to react to the slaughter of one of their gods. The Israelites, of course, didn't know this, and therefore displayed tremendous faith in Hashem prior to Yetziat Mitzrayim (the Exodus). We remember this event with a special Haftorah, Malachi 3:4 - 24, where again great faith and trust in Hashem is emphasized. The Haftorah concludes with the call to remember the teachings of Moshe and informs us that Hashem will send Eliyahu Hanavi (Elijah the Prophet) to herald the great and awesome day when Bnei Yisroel will again experience redemption. This is yet another possible reason for the name Shabbat HaGadol, - that "great day" mentioned in the Haftorah.
Traditionally on Shabbat HaGadol the Rabbi lectures about the observance and meaning of Pesach to his congregation, teaching the laws of Pesach, so that the families can prepare properly for the Yom Tov. Which leads to another interpretation of Shabbat HaGadol - "the Shabbat of the Leader" or of the Rabbi. A more novel explanation is that the people returning from the synagogue later than usual on this Shabbat because of the unusually long speech that was customary on this day. Thus this Shabbat seemed "great," i.e., longer than the other Shabbatot.
Whatever the reason for the name, it is customary to recite part of the Haggadah on Shabbat HaGadol, from " עבדים היינו - Avadim Hayinu" we were slaves in Egypt" to " לכפר על עוונותינו Lechaper Al Kol Avonoteinu."
HAFTORA: (Additional portion, from Prophets, which is read after the Parsha.)
Malachi 3:4 - 24 repeating verse 23 at the end.