Day of the Week Panties refer to women's underwear/panties that correspond to the days of the week that became popular in the 50's most especially to teenagers. "Day of the Week Panti es" was named as such since each underwear/panty has a different color for each day of the week (7 colors) and was embroidered with a separate day of the week, corresponding to seven ( 7) panties.
The "Day of the Week Panties" hit their popularity heights in the 1950's, but they survived in some catalogs and in some stores.
Author's Note: I can not remember this trend, however, I remembered that my mom used to buy me and my other 2 sisters panties in 7 colors corresponding to the day in which we should wear them depending on what color we want to wear for what day and until now I carry this tradition.