This is a reminder to make an eruv tavshilin. This year, we make an eruv tavshilin on erev Rosh Hashana, on erev Sukos, as well as on Hoshana Raba (erev Shemini Atzeres). Whenever Yom Tov falls on Friday, we make an eruv tavshilin the day before Yom Tov in order to allow us to cook on Friday for Shabbos. The way to make an eruv tavshilin is to take a choshuv, important, cooked food (such as a piece of meat, chicken, fish, or an egg) and preferably a baked food as well (for example a challah), put them together, pick them up and lift them at least a tefach, handbreadth, and say the text of eruv tavshilin as written in the siddur. If a person forgot to make an eruv tavshilin, they cannot cook or light candles on Yom Tov for Shabbos. If they remember on the first day of Yom Tov, there is a particular tnai, stipulation, that can be said to allow making an eruv tavshilin on the first day of Yom Tov. However, this tnai is not applicable on the first day of Rosh Hashana. There are other options available as well to make an eruv tavshilin if a person forgot. One option is to rely on the Rov in the city who makes an eruv for all residents of the city. However, this depends on whether the person was negligent or the forgetfulness was due to an accident. In fact, some things that we might consider accidents are halachically considered negligence, and that is something to discuss with a Rov. Even with an eruv tavshilin, a person has to make sure that they only use the eruv tavshilin to prepare food on Yom Tov for Shabbos which includes washing dishes and lighting candles, but not other preparations for Shabbos. All food must be prepared well in advance on Friday, in order that it could be eaten already on that day well before Shabbos begins.