yom tov - If you forgot to make an Eruv Tavshilin

I recall hearing that if you forgot to make an Eruv Tavshilin there is a way to make it on the first day of Yom Tov. Does anyone know how this is done?

Answer

Shulchan Aruch (Orach Chaim 527:22) says that you should make the following declaration when making an Eruv on the first day of Yom Tov:

"If today is a Yom Tov, then I don't need an Eruv [because tomorrow is really a weekday, and I can prepare for Shabbos as usual]. But if today is a weekday, then with this Eruv I shall be allowed to bake, cook, ...."

Note the following caveats:

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  This doesn't work for Rosh Hashanah, because its two days are halachically considered "one long day."

  
  


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  According to one opinion cited in Shulchan Aruch there, this works only if you have some food already prepared from before Yom Tov. (Though Mishnah Berurah, ibid. :75, says that in practice we don't follow this opinion.)

  
  


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  There are variant opinions as to whether to recite the blessing when making an Eruv with this condition (Mishnah Berurah ibid. :74).