Prisoners are likely really depressed all the time but when it comes to holidays such as Thanksgiving the depression is likely worsened.
I've heard of prisons serving a Thanksgiving meal that is more than the regular prisoner meals they serve on a daily basis.
Usually the prisoners will get some Turkey, mashed potatoes, rolls, possibly ham and other good Thanksgiving food which can help keep the prisoners spirit up.
Not all people in prison are terrible people even though they have done bad things and some prisoners can turn their life around so they deserve a second chance.
The prisoners don't get a Lavish Thanksgiving dinner and cannot eat all they want but they do get a better meal when it comes to Thanksgiving which can help the prisoners feel a little better and give them hope should they get released from prison.