אך את שבתותי תשמורו (כי תשא לא:טז)

On the passuk (Devorim 31:22) “אך את הזהב” Rashi says that before Kosherizing a vessel one has to clean it from dirt so that only the gold remains.  

The same applies to the holy day of Shabbos:  Before a person approaches the holy Shabbos day, he needs to clean himself from the dirt accumulated during the week.

אך את שבתותי תשמורו – he will be left with the purity of Shabbos – to be able to observe the holiness, cleansed.