Sunday nigger 

A white person who acts white among white peers and family, but reverts to acting black in company of black people.

That white boy just a Sunday nigger

Sunday nigger by Pizzlej December 5, 2021

Niggar Sunday 

Monday. Also any day of the week that a black person opts not to go to work/school.

Chuck:"Where's Jamal, I haven't seen him around the office."
Dave:"Understandable, it's niggar sunday."

Niggar Sunday by Zacheryy January 22, 2007