As we have seen, there is a mitzva to add from the mundane to the sacred and to accept the holiness of the day before shki’a. This acceptance needs to be verbalized. There are two customs regarding when this is done in synagogues, based on when Kol Nidrei is finished.
Some congregations make sure to finish Kol Nidrei before shki’a, as there is a principle that we do not annul vows on Shabbat unless doing so meets some Shabbat need (Shabbat 157a). Since this principle applies to Yom Kippur as well, Kol Nidrei should not recited on Yom Kippur itself. True, some maintain that Kol Nidrei is primarily meant to neutralize future vows (5:12 above) and thus is not considered nullification of vows; nevertheless, others maintain that it is similar to the annulling of vows, and therefore should not be recited once the day has begun (Rema 619:1; MB ad loc. 5). These congregations should accept the day’s holiness with the recitation of She-heḥeyanu at the conclusion of Kol Nidrei. Since this berakha expresses our thanks for this day, it is an appropriate time to accept the day’s sanctity. Even though on other holidays we recite She-heḥeyanu over wine at kiddush, on Yom Kippur, when we do not recite kiddush over wine, we recite She-heḥeyanu at its onset (Eruvin 40b; SA 619:1).
Most congregations, however, finish Kol Nidrei after shki’a, with some even starting it after shki’a. Even though the halakha is that we do not annul vows on Shabbat, when it meets a Shabbat need it is permitted. Since the recitation of Kol Nidrei is meant to cleanse us of the sin of unfulfilled vows, it meets a Yom Kippur need. (See Shabbat 157a; SA 341:1.) These congregations must accept the day’s holiness before shki’a to fulfill the mitzva of extending the day’s sanctity. To ensure that people do not forget to fulfill the mitzva, the gabbai should announce: “We hereby accept upon ourselves the sanctity of Yom Kippur.” Sometimes the ḥazan thinks he will reach She-heḥeyanu before shki’a, but during the recitation of Kol Nidrei it becomes clear that he will not. In such a case, he should pause before shki’a to announce that the holiness of the day is being accepted, and then continue his recitation.