The idea of waiting until tzeit hakochavim to start Shavuot is not found in Chazal; it arises first in the works of early Acharonim, beginning with the Masat Binyamin.[2] The matter is based on the idea that since Shavuot follows a 49-day period, it should not begin before that time period has finished. The problem could be that it is not yet time for Shavuot; alternatively, although one may start Shavuot early per se, in so doing one is improperly “shortchanging” the omer period.
Early sources[3] focus on the pasuk, “They shall be seven complete (temimot) weeks,”[4] positing that beginning Shavuot early impinges on the completeness of the omer period, since one is ending the period before it is fully complete. (This is likely related to the idea that one should count promptly on the first night of the omer due to the need for temimot, i.e., so as to begin the seven-week period of counting close to the weeks’ actual beginning.[5]) The Netziv,[6] however, sees in the pasuk regarding Shavuot, “You shall call, in the midst of this day, a holy convocation,”[7] an indication that it is specifically that Shavuot should not start before nightfall. L’Horot Natan[8] prefers the earlier sources, which relate the matter to the impact on sefirat ha’aomer rather than to when Shavuot begins. He claims that as a result, one who made Kiddush early fulfilled his mitzva of Shavuot, just as he would at that time of day for Shabbat and other yamim tovim, and he need not repeat Kiddush.
Regarding which Shavuot element(s) should one wait? The Masat Binyamin,[9] Shelah,[10] and Magen Avraham[11] refer only to Kiddush, and the Shelah writes explicitly that Ma’ariv (and Kiddush in shul) may be recited earlier. The Taz[12] maintains that one should wait for Ma’ariv as well. The Pri Megadim[13] supports the Taz by pointing out that in the context of several halachot, Ma’ariv ushers in a new day. The Mishna Berura[14] posits that one should indeed wait until tzeit hakochavim to daven Ma’ariv, and this is the widespread minhag. (Hitorerut Teshuva[15] supports the delay of Ma’ariv for an additional reason: On Shavuot, many people remain awake all night and do not recite Kri’at Shema Al HaMita, and it is therefore important for the Ma’ariv Kri’at Shema to be recited after tzeit hakochavim.[16])
What about candle lighting? It is permitted to light candles on Yom Tov itself (with certain conditions that do not exist at a pre-Shabbat lighting), and lighting before sheki’a usually ushers in the holy day in whose honor the candles are lit.[17] For these reasons, it would seem logical to compare candle lighting to Kiddush and Ma’ariv and to therefore require waiting to light until tzeit hakochavim. However, while there is indeed an opinion that one should wait until tzeit hakochavim to light the candles, the minhag is to light them at the regular Shabbat/Yom Tov time.[18] One explanation for the common practice is that lighting candles does not constitute an absolute acceptance of Shabbat or, in this case, of Shavuot. One could also claim that it is permitted for women, who are the ones who accept Shabbat with lighting,[19] to “ruin temimot” of sefirat haomer, because they are exempt from counting the omer in the first place. However, L’Horot Natan[20] argues powerfully that this application of temimot is not a function of the mitzva to count, but of respecting the time-period’s integrity, and that principle applies to women as well.
The main reason not to preclude lighting candles for Shavuot before tzeit hakochavim is that there probably is no intrinsic problem at all. One violates no mitzva by performing an act of Shavuot early, and the omer period is one of seven complete weeks irrespective of any Shavuot actions we might take. Chazal, who are our source of formal limudim, are, in fact, silent on the matter. Rather, the matter of waiting, which we do not find regarding Shabbat or other yamim tovim, is a minhag instituted because it looks like (language of the Shelah) we are shortchanging the omer period. Thus, there is no innate need to separate between the period of the omer and Shavuot, but only an expectation that we should act according to the extent of care the minhag prescribes. There is a consensus regarding Kiddush to wait for tzeit hakochavim, and the practice has extended for most to Ma’ariv; however, no more than a few apply it to candle lighting.
Because many classical poskim allow early Ma’ariv on Shavuot, it is legitimate for a shul/minyan with a special need to do so, despite the prevalent minhag. If feasible, it should be emphasized that only those with a real need attend such a minyan. (It is unclear if accommodating children’s sleep patterns qualifies as such a need.) It is proper to communicate that all who can should not make Kiddush until (the earliest opinion of) tzeit hakochavim. Even if the rabbi feels that some people are reciting Kiddush before tzeit hakochavim, the congregation may continue davening Ma’ariv somewhat early, unless it is known that there is widespread “abuse.”