The Taz in Orach Chaim 494 writes that we delay davening Maariv on Shavuos night so that the counting of the Omer should be complete. The Magen Avraham 494 says that on the night of Shavuos we don’t make Kiddush until after tzais hakochavim so it should be temimos. The question can be asked; is it permissible for women, who are exempt from the mitzvah of Sefiras HaOmer because it is a time bound mitzvah, to daven Maariv earlier or make Kiddush early? Is the concept of temimos necessary in order to bring in Shavuos?
The Teshuvos Lehoros Nosan Vol 7:31writes that women, despite the fact that they are exempt from Sefiras HaOmer, are still required to wait and not make Kiddush until after tzais hakochavim. The reason for this is that the Yom Tov of Shavuos is different than any other Yom Tov in that the time of the month is not explicitly written in the Torah, whereas regarding all other Yomim Tovim, the Torah specifies a date. Shavuos is identified as being seven weeks after the second day of Pesach. Since the Torah identified this Yom Tov as being seven weeks after Pesach, it must be complete weeks and therefore even women would be required to wait for Kiddush so as to bring in the Yom Tov on time. This would seem to be a big nafka mina for the women. Normally, on the first day of Yom Tov, women light while it is still day and are mekabel Yom Tov early, hereby enabling them to make the bracha of shehechyanu. On the night of Shavuos it would behoove the women to light later on so the bracha of shehechyanu is said at the right time.
Another question that can be analyzed is; what would be if somebody forgot to make the bracha of shehecheyanu on Shavuos, would he have seven days to make it up - like korbanos have the possibility of tashlumin for seven days? The Chok Yaakov 473:1 and Magen Avraham 473:1 both agree if a person forgot shehecheyanu on the first day of Yom Tov, he can make it up in the next seven days. The Maharil Diskin in 5:54 asks the following question on the Chok Yaakov. The gemara in Eiruvin 40b asks whether or not on Rosh Hashana and Yom Kippur one can say shehecheyanu despite the fact that it is not a Regel? If so, then the seven days after Shavuos is definitely not a Yom Tov and if one would celebrate it as a Yom Tov, he would be oveir on Bal Tosif. How is it that he can make a shehecheyanu on those days, especially in this day and age when we don’t bring any korbanos?
We could answer that since during this time one would have a mitzvah of visiting his Rebbe, the simcha of greeting his Rebbe would warrant one to make the bracha of shehecheyanu. What would result based on this tirutz is that if someone forgot to say shecheyanu on Shavuos, he would have to go visit his Rebbe and thereby make-up the bracha of shecheyanu.
May we be zocheh to be mekayem the mitzvah of greeting our Rebbe in person in our schools and Batai Medrash together with Moshiach Tzidkeinu and say the bracha of shehecheyanu with newfound kavana!