Exode 19:10

And the Lord said to Mosheh on the fourth day, Go unto the people, and prepare them today and tomorrow; let them wash their raiment,

V’KIDASHTAM’ TODAY AND TOMORROW. Rashi explained: “V’kidashtam means ‘and thou shalt prepare them.’” And so is the opinion of Onkelos. A similar usage is found in the verse: I have commanded ‘lim’kudashai’ (“those who are prepared for Me” or “My consecrated ones”), and also in the following verse: ‘hithkadshu’ (prepare yourselves) for tomorrow. Rabbi Abraham ibn Ezra explained it as meaning that they should bathe themselves in water. But if so, what is the sense of today and tomorrow when bathing was required only once? The correct interpretation is that they should sanctify themselves by separating from their wives, and from all uncleanness, for he who guards [himself] from being defiled by uncleanness is called ‘m’kudash’ (consecrated), just as it is said with reference to the priests, There shall none defile himself for the dead; They shall be holy unto their G-d., Verse 6. And it is further written, because the priests had not sanctified themselves, that is to say, they had not purified themselves. Similarly, And David answered the priest, and said unto him: To a certainty women have been kept from us about these three days; when I came out, the vessels of the young men were holy. And it is known that [before the Giving of the Torah the people] bathed themselves in water, this being derived logically from the required washing of garments., as Ibn Ezra did, as meaning that they shall bathe themselves in water, for that requirement can be derived from the express commandment, and let them wash their garments (here, Verse 10). Hence, v’kidashtam must mean as Ramban explained it. The reasoning for the requirement of bathing or immersion is found here in the Mechilta: “And let them wash their garments. And whence do we know that immersion was also required? I reason as follows: If immersion is required in cases where washing of garments is not required (see Leviticus 15:16), is it not logical that immersion is also required in this case where washing of garments is expressly mentioned?” And so did the Rabbis say in the Mechilta: “There is no case where washing of garments is required in the Torah without the requirement of immersion [of one’s body].”

וקדשתם, a way of saying “prepare them.” You have the same word in the sense of preparing for an event which did not require ritual purity as prerequisite in Numbers 11,18 before the people would receive meat.