National Fat Week is when Garfield celebrates being fat and telling "skinny jokes" (making fun of people who are not fat and try to stay that way). This has appered three times in the strip, first in September 1979 then in 1980 in the same month, and finally in January 1985.
National Lazy Week
There was also National Lazy Week, which ran from January 5 to January 10, 1987. As the name suggests, it celebrated being lazy.