Almost everyone uses the acronym TGIF on Friday but not on other days of the week. The most common meaning of TGIF is "Thank God It's Friday!" even though some people use it to mean "Thank Goodness It's Friday."
So, why do they say it on Friday even though Friday is still a workday?
Whether people love their jobs or not, they still enjoy having the weekend off. Once they make it to Friday, they thank God for having made it through the previous four days with the assumption they can make it this last day also.
When people say TGIF, they are grateful that Friday is the last day of the week and the beginning of the weekend when they can take a short break. Then they will have extra time to spend with the family, to do work around the house, or to stay up late at night without having to worry about getting up early to go to work the next day.
To say TGIF doesn't mean you don't love your job. Instead, it could be thought of as a short prayer to God as you thank Him for His protection and provision for having kept you throughout the week. Also, it is a prayer in advance for the upcoming break you are about to take on the weekend.