Dear Sholem,

I didn't find an explicit source to answer this yet. But it is a very interesting question! On Erev Shabbos they needed a double portion of Manna for a combination of two reasons:

#1) No Manna would fall on Shabbos; and

#2) They needed to eat meals on Shabbos.

On Yom Kippur, consideration #1 indeed applies, since no Manna would fall on Yom Kippur [1]. But consideration #2 might appear to be irrelevant, since we are not supposed to eat anything on Yom Kippur. Therefore, one might argue that no extra portion of Manna should be necessary on Erev Yom Kippur.

On the other hand, there typically are people who need to eat on Yom Kippur for health reasons; and so an extra portion of Manna might be appropriate for those special cases. However, even that arguemnt is definitive, since those who needed to eat could alternatively partake of other available foods [3].

By the way, some commentaries even ask the question: If our Gemara considers eating Manna to be an affliction, then it should be permitted for anyone to eat it on Yom Kippur; however, others resolve this proof, by saying that the affliction was only referring to the hunger each day before the Manna fell [2].

I hope this helps.

Best wishes,

Yishai Rasowsky

Notes:

1. A number of statements in Midrashim, for example Mechilta, can be found in this link https://www.sefaria.org.il/Exodus.16.26?lang=bi&with=Midrash&lang2=en

2. https://dafdigest.org/masechtos/Yoma%20074.pdf and the third paragraph in the left column here https://hebrewbooks.org/pdfpager.aspx?req=14522&st=&pgnum=121&hilite=

3. E.g. cattle meat https://www.sefaria.org.il/Yoma.75a.21?lang=bi&p2=Rashi_on_Yoma.75a.21.1&lang2=bi

Dear Sholem,

One could suggest other potential reasons why the Manna might have fallen on Erev Yom Kippur, even for those who would be fasting.

First, to provide extra food for the Mitzvah of eating on Erev Yom Kippur [4] in greater amounts than on a regular day [5].

Second, to ensure that food was available when the fast would be over. This Manna could not have fallen on Yom Kippur itself [1]. It also would not be reasonable to expect that it fell the following evening, since it typically was resting between the layers of dew [6].

Best wishes,

Yishai

Sources:

4. https://www.sefaria.org.il/Yoma.81b.4?lang=bi&with=Rashi&lang2=en

5. https://www.sefaria.org.il/Shulchan_Arukh%2C_Orach_Chayim.604.1?lang=bi&with=Mishnah%20Berurah&lang2=en

6. https://www.sefaria.org.il/Yoma.75b.9?vhe=William_Davidson_Edition_-_Vocalized_Aramaic&lang=bi&with=Rashi&lang2=en