QUESTION: May I perform melacha during Bein Hashemashos (twilight) after the first day of Yom Tov, for the purpose of deriving benefit from the melacha during the same time frame of Bein Hashemashos?

ANSWER: It is uncertain whether Bein Hashemashos, the period from sunset until nightfall, is considered day or night. Moreover, that uncertainty extends to every instant of Bein Hashemashos; perhaps a particular moment is day (part of the preceding day) but at the very next moment nightfall has commenced, and the new day has already begun. Hence, even if a person performed a melacha and benefited from it immediately, there is the possibility that the melacha was performed on the first day of Yom Tov and the benefit from it was derived on the second day of Yom Tov, and it would accordingly be forbidden.

However, R’ Akiva Eiger (Ginzei R’ Akiva Eiger 21) suggests that there is a double uncertainty (a sefeik sefeika) regarding melacha during Bein Hashemashos: Perhaps the melacha was performed on the previous day and the benefit was derived then as well; and even if it was already night when the benefit was derived, perhaps the melacha was also performed at night. R’ Akiva Eiger concludes that for Biblical melachos, such as cooking and carrying in a public domain, this double uncertainty should not be relied upon (for various reasons). However, with regard to Rabbinic ordinances, one may rely on this sefeik sefeika, provided that the benefit is immediate.

Accordingly, one may carry a siddur to shul, during Bein Hashemashos, if two caveats are met: First, the place through which he walked is an area where it is prohibited to carry on a Rabbinic level (i.e., it is a karmelis and not a Reshus Harabbim), and secondly, he arrives in shul and uses the siddur while it is still Bein Hashemashos. The consensus of contemporary poskim is to follow R’ Akiva Eiger’s approach (Yom Tov Sheini K’hilchaso Chapter 1, footnote 21).

Rav Schachter, Shlita, said that those who have the custom to make early Yom Tov on the second day of Shavuos should accept Yom Tov as early as possible (i.e. immediately after plag haMincha).  In this way they can avoid the concern of preparing from one day to the next since the food will be cooked and served before Bein Hashemashos.