In this week's Parsha we are taught that Aharon was prohibited from entering the Holy of Holies at any time of his desire. Rather, he was only to enter on Yom Kippur and with the prescribed service that is mentioned. The question one might ask is why would Aharon think to enter at a different time, for what purpose would he be going in?
Rav Yehonasan Eibshitz offers the following understanding of this segment of the Torah. Yom Kippur is the tenth of the month of Tishrei which is the month in which the world was created. However, the Gemara mentions that Rebbi Yehoshua's position is that the world was actually created in Nisan and not Tishrei. If so, one could have thought that it is necessary to enter the Holy of Holies and perform that Yom Kippur service on the tenth of Nisan, as well. Therefore, the Torah specifies that it is only in Tishrei that the service is to be performed. Part of the reason for this is that on the first day that the Mishkan was erected (which was in Nisan), Nadav and Avihu died. Their death, recorded at the beginning of this week's Parsha, is considered to bring atonement as the death of the righteous brings atonement. Thus, their death is in lieu of a the Yom Kippur service that would have been performed later that month. (Tiferes Yehonasan)
Interestingly, on the tenth of the month of Nisan, which would be the Yom Kippur, Bnai Yisrael did experience a miraculous event. That was the day that they took their first Korbon Pesach in Mitzrayim many years ago. It is for that event that many have suggested that this week is called Shabbos HaGadol.