While many people choose to begin Shabbat earlier than required in the summer months, there is a general hesitation to do so on Shavuot. Rather, common custom is to wait until nightfall before reciting maariv or kiddush. The reason for this custom is because the omer period, the period between Pesach and Shavuot, must be “seven complete weeks.”
It is believed that beginning the holiday of Shavuot before nightfall, namely, before fifty full days have passed since Pesach, renders the seven-week period incomplete. It is also noted that, with regards to Shavuot, the Torah uses the term “b’etzem hayom hazeh”, on this very day, which seems to imply that Shavuot must be observed precisely when it is intended to be, and no earlier. For these reasons as well, many women wait until after nightfall to light the Shavuot candles even though they normally light the Yom Tov candles before sunset at all other times, just like the Shabbat candles. As the summer days are nearly at their longest around Shavuot, waiting until nightfall to begin Yom Tov makes for a very late start to the Yom Tov meal.
Nevertheless, there are many authorities who rule that the requirement to wait until nightfall applies only to the recitation of kiddush. It would be permissible, however, to recite maariv before this time. Doing so would assist in being able to begin the Yom Tov meal that much earlier. Indeed, according to this approach one would also be able to fulfill the mitzva of Tosfot Yom Tov by reciting maariv slightly earlier, and then simply waiting until nightfall to recite kiddush and partake in the Yom Tov meal.
There is also a school of thought which subscribes to the idea of reciting maariv before nightfall although they recommend waiting until at least after sunset to do so. In contrast, there is also a view that the mitzva of Tosfot Yom Tov is waived on Shavuot in deference to the more explicit mitzva of ensuring “seven complete weeks”. According to this approach, there would be little justification for reciting either maariv or kiddush before nightfall.
It might just be that the custom to delay the recitation of maariv until nightfall was instituted simply in order to ensure that one would not come to inadvertently recite kiddush before this time. Indeed, there was some concern that one who recited maariv early might forget himself and proceed to recite kiddush immediately thereafter, as well.
Another reason offered for delaying maariv is related to the custom of staying awake all night long on Shavuot. A number of authorities expressed concern that those who remain awake all night long might forget to repeat the Shema as is required in the event maariv is recited before nightfall. This is because those who recite maariv early all year long are in the habit of repeating the Shema as part of the Shema recited before going to bed. Since this routine is obviously interrupted on Shavuot night, one might forget to repeat the Shema entirely.
Finally, R’ Gil suggests that the reason Shavuot is not started early is because it is easier to stay up all night immersed in Torah study when the study session begins late, following a late Yom Tov meal. Finishing the Yom Tov meal early in the evening, however, might make for an uncomfortably long study session.
*********************************************** Mishna Berura 494:1
 Taz O.C. 494:1
 Tzitz Eliezer 13:59
 Piskei Teshuvot 494:2, Lehorot Natan 7:31
 Magen Avraham 494
 Daat Torah 494:1
 Biur Halacha 261:2, Avnei Nezer O.C. 316:12
 Siddur Beit Yaakov;Shavuot
 Melamed L’hoil 1:108
 Rivevot Ephraim 8:491:5
 Lehorot Natan 7:31
 Hitorerut Teshuva 2:56