Why does Douglass assert that this practice is "one of the grossest frauds committed upon the downtrodden slave"?

Douglass asserts that the holidays are "one of the grossest frauds committed upon the downtrodden slave." The reason he gives is that the holidays were presented as a wonderful thing, born out of benevolence of the master towards the slave. Instead, holidays were only implemented as a way to make slaves disgusted with freedom. Slaveowners pushed slaves towards drinking excessively throughout the holidays, and therefore, feeling sick and disgusted with themselves. It also convinced them of the benefits of slavery, as it kept them sober and working, so that when the holidays were over, they were happy to get back to work.