Needless to say, Moshe is the central, most dominant, figure in the Torah; after all, he wrote it as he heard it from God. Four out of the five books of the Torah deal with Moshe, from his birth in Egypt until his death on the eastern side of the Jordan river. So it is not surprising that Moshe appears in almost every chapter of the Torah, beginning with Shemot and ending in VeZot HaBeracha. Curiously, he is absent from this week's parasha. The name "Moshe" does not appear in this shabbat's reading.
The Ba'al Turim explains this absence by referring us to next week's parasha and the story of the golden calf, the egel. In face of God's fury, Moshe prays and pleads with God not to punish the people of Israel for their abominable sin, and he adds a condition: either you, God, forgive them or else you erase my name off the book you wrote, the Torah. Moshe tells God that if He is not willing to pardon Israel for their sin, he does not want to be mentioned in the Torah that God gave Israel. God accepts Moshe's prayer and responds by saying that Moshe's name will not be erased; the names of those who sinned will.
However, since we have a rule that a curse placed by a great sage does not go unfulfilled, Moshe's threat had to be accomodated in some way. And the way was to "erase" his name from one of the parashot in the Torah, and that's the reason why tomorrow we will not mention Moshe's name in the reading.
But why this week and not any other week? Tetsave is almost always read during the week of the 7th of Adar (today is the 9th), which is the date of birth and death of Moshe. We know that Moshe died on this date by calculating back from the date the people of Israel entered the promised land, as described in Yehoshua (it was the 10th of Nissan, after 3 days' work by the spies in Jericho plus 30 days of mourning after Moshe's death, hence the 7th of Adar). And we know this was also his birth date as sages always "complete" their years, and Moshe lived exactly 120 years.
So that is the reason Moshe is absent from this week's parasha. Although we do not have the custom of mourning the death of Moshe on the 7th of Adar, we nevertheless have a sign to mark this date: the absence of his name. It is a reminder both of his unfulfilled curse and of the date of his passing away.