The holiday of Shavuot begins Thursday evening. Known in English as Pentecost because it takes place fifty days after Passover, Shavuot commemorates the giving of the Torah on Mount Sinai. The two main customs associated with the holiday are serving dairy foods and staying up all night to study Torah. Staying up all night only became popular once coffee became readily available in Europe.
This year the one-day holiday of Shavuot leads right into Shabbat. Cooking is permitted, but only for the holiday itself. This creates a problem with heating or cooking food for the Sabbath, when all cooking is forbidden. To get around this we put aside some cooked food on Thursday, before the holiday begins, make a blessing and designate it for one of the Shabbat meals. It’s as if we begin cooking officially for Shabbat before the holiday actually started. This food is known as an Eruv Tavshilin.
In Israel all yamim tovim—the holidays with this restriction—are observed for only one day with the exception of Rosh Hashanah. Outside of Israel every yom tov is two days long. Judging by the crowds, it seems that many Israelis panic at the thought of stores closing for two days in a row. A one-day observance also means that making an Eruv Tavshilin is rare. Here the only holidays that can fall on Fridays are Shavuot, the second day of Rosh Hashanah, and the last day of Passover, and that doesn’t happen even every year.
Have a chag sameach and Shabbat shalom.