In the entire Parshas Tetzaveh, the name of Moshe Rabbeinu is not mentioned once. This is highly unusual, for ever since the Parsha that tells of his birth, there are very few other places in the five books of the Chumash where he is not mentioned, and there are none until the book of Devarim. His absence from our Parsha is explained as being the fulfillment of Moshe's request, following the sin of the golden calf, when he asked Hashem to please forgive Bnai Yisrael's transgressions and if not, to please blot him out from the book which Hashem has written (שמות ל"ב:ל"ג). Although at that time Hashem did indeed forgive Bnai Yisrael, the Gemara in Makkos (דף י"א.) teaches that the declaration of a Tzaddik is fulfilled, at least in part, regardless of any conditions he may have included therein. Consequently, Moshe's name was "blotted out" from part of Hashem's book, meaning, of course, the Torah, and thus his name does not appear in this Parsha.
Now that we have explained why Moshe's name had to be excluded from some portion of the Torah, we are still left with the question of why now, in this particular Parsha. Some explain that this Parsha is usually read during the week in which the seventh of Adar, which is Moshe's Yahrtzeit, occurs; we thus omit his name from the Parsha that is read near the date of his death, perhaps to highlight his absence. Others, however, also address this question and answer in the following manner. The entire Parshas Tetzaveh is in itself actually a continuation of the description of the details necessary for the building of the Mishkan, which began in Parshas Terumah. As such, Parshas Terumah and Parshas Tetzaveh can actually be considered as a single entity, the latter being merely a continuation and extension of the former. If we understand the two Parshiyos in this way, we can see why Parshas Tetzaveh would be the optimum Parsha in which to leave out Moshe's name. Since Moshe's name is mentioned several times in Parshas Terumah, and Parshas Tetzaveh is simply an extension of Parshas Terumah, then we are really able to say that Moshe is actually included in virtually every Parsha after his birth, while still fulfilling his declaration by excluding him from Parshas Tetzaveh proper.
We can extend this understanding of these two Parshiyos to a dispute between two early sources concerning the total number of weekly Parshiyos in the Torah. One authority claims that there are only 53 Parshiyos, while another indicates a total of 54. Some explain this discrepancy as being due to the first authority's exclusion of Parshas VeZos HaBeracha from the count because it is read on Simchas Torah, not Shabbos, and it therefore is not counted as one of the weekly Parshiyos. However, with our understanding of these two Parshiyos as mentioned above, we can say that perhaps the first authority arrived at his total of 53 by counting Terumah and Tetzaveh, as we have established, as one Parsha, while the other authority counted them as two.