EATING MEAT ONLY ON SHABBOS - Yevamos 90 - Daf Yomi

The Mekubalim explain the custom of some Chassidim as to why they would not eat meat during the week. Our Gemora said that when the Kohanim eat the meat from a sacrificial offering, the owner receives atonement. There are wicked people who after their death. Their soul enters into an animal as a gilgul. When a person consumes this animal, the soul of the wicked person intermingles with the soul of the person who ate the animal. When he sleeps at night and the soul ascends to the Heaven, the two souls separate. The soul belonging to the wicked person remains in Heaven and receives atonement. This is what is meant that the owner receives atonement. This process could only be accomplished together with the sanctity of Shabbos. This also explains the opinion of Beis Shamai. When they saw an animal that had a spark of sanctity contained within it, they would save that animal for Shabbos.

Sefer Chemdas Yamim