There is a question among the Poskim regarding the recital of Arvit and Kiddush on the eve of Shavuot before Tzet Hakochavim, nightfall. In many locales in the northern hemisphere, nightfall is very late at this time of the year and praying and reciting Kiddush at that time may not be pragmatic. However, in describing Sefirat HaOmer, the Torah says “Sheva Shabbatot Temimot Tihyena” (Vayikra 23:15); which means that the Sefira must comprise seven complete weeks. By reciting Arvit and Kiddush (which declare the onset of Shavuot) before nightfall, it follows that Arvit and Kiddush are being recited on the 49th day of the Omer and that the seven weeks are not complete.
The Magen Avraham (Siman 494) cites the Masat Binyamin, who says that Kiddush should not be recited before nightfall because it will be taking away from a portion of the last day of the Omer, which is not in keeping with the principle of Temimot. The Taz says that even Arvit should not be recited prior to Tzet Hakochavim. Rabbi Yaakov Emden, however, states in his Siddur that in time of need one may be lenient regarding both Arvit and Kiddush.
Rabbi Ben Zion Abba Shaul (Or LeTzion vol. 3, 18:4) rules that Lechatehila (ideally) both Arvit and Kiddush should be recited after Tzet Hakochavim. Where necessary, however, such as in cities with very late nightfalls, he rules that one may recite both Arvit and Kiddush before this time. Rabbi Shimon ibn Danan, the former Chief Rabbi of Morocco, used to pray Arvit only after Tzet Hakochavim. It should be noted that Rabbi ibn Danan’s practice took place in a location where nightfall occurs at a relatively early hour, and it is unclear if this custom would apply to other cities as well.
This should not be an issue this year (5775/2015) as Shavuot falls on Motzaei Shabbat and Arvit and Kiddush will, by necessity, take place after Shabbat ends and Tzet Hakochavim.
Summary: Ideally, both Arvit and Kiddush of Shavuot eve should be recited after Tzet Hakochavim. If this is not practical due to the late hour, both may be recited before that time. Tzet Hakochavim for your location may be found here.