Normally, the concept of a “3-day Yom Tov,” in which Yom Tov and Shabbos come in succession, is only something that happens outside Israel.
Rosh HaShana is the exception to the rule because it is the only Yom Tov that spans two days, and can then lead into Shabbos.
When this happens, we make an Eruv Tavshilin. This is a Rabbinic Enactment which involves taking a cooked food and a baked food and setting them aside for Shabbos before Yom Tov. A bracha is made and a declaration recited.
This permits preparing food on Yom Tov for Shabbos, which had been prohibited by the rabbis lest people focus on Yom Tov and forget about preparing for Shabbos, and also lest they prepare on Yom Tov for the next day or a weekday.
When the whole world must fulfill this mitzvah, it behooves us to consider that this world is also intended for us to prepare for the next world, which is considered “a day entirely of Shabbos.”
We prepare here, and in Olam HaBa (including when Moshiach comes), we will enjoy what we’ve prepared.
The items we need are the Torah and Mitzvos we can learn and do in this life, and we should recognize that all we’ll have then, is what we prepare now.
