el sábado, sabadete, camisa nueva y polvete

Etymology
lit. 'Saturday, Saturday, new shirt and powder'. Traditionally, before the days of washing machines, men would wash their shirt only once a week (Saturday). The polvete (powder) refers to the talcum powder used to clean the shirt. The word sabadete may have originally been a nonce word coined in order to rhyme with polvete.

Over the years, polvo has acquired a slang meaning of sexual intercourse, so the phrase has come to carry connotations of weekend sexual liaisons.

Phrase

 * 1)  ; thank God it's the weekend