Final answer:
From sunset until dark at the end of the first day of Yom Tov, it is forbidden to start any activities classified as work due to the transition from the holy day to a regular day.
Step-by-step explanation:
At the end of the first day of Yom Tov, from sunset until dark, it is forbidden in Judaism for activities that are considered work to be initiated. This period is known as beit hashmashot, and it is a time when the distinctions between the holy day and the regular day are blurred. Hence, activities such as writing, cooking, or turning on electricity, which are restricted during Shabbat and major Jewish holidays, should not be started until after full darkness to ensure that the sanctity of the holiday is maintained. The rationale behind this prohibition is to extend the honor of the Yom Tov and avoid encroaching on its holiness with everyday work.