Responsa for Beitzah 33:11

Q. Rosh-Hashanah fell on Thursday and Friday. On Wednesday A forgot to perform the ceremony of Erub-Tabshilim [which would have enabled him to prepare food on the holiday for the Sabbath]. May he perform the ceremony on Thursday with the condition that if Thursday is Rosh-Hashanah, and Friday is a week-day, the ceremony is void, but if Friday is Rosh-Hashanah, and Thursday is a week-day, his ceremony of Erub-Tabshilin will be valid. [This is based on the assumption that Rosh-Hashanah, like all other holidays, really consists of one day, but that we observe two days not knowing exactly which day it is.]
A. No, but he is permitted to cook on Rosh-Hashanah, for the Sabbath, just enough for his bare needs. Moreover, he may cook and bake on the holiday more than he needs for the holiday, and eat the remainder on the Sabbath.
SOURCES: Pr. 317.