Even has a few meanings in the Bible. "In the even", "Even as", "This Even" and many other meanings. but this one that has caught the attention of so many lest clear that up. The Phrase "until the even" EVEN is the same as END. So saying "Until the even" is until your end. Stating the importance of being cleaned before going on with your days.
For those that would use any version of the bible not the KJV. I would warn you that the translators did not fully understand what they were reading. you did not become clean just because it was evening and you had washed in water. No it is the soul that is unclean until the even (END). Unless you repent you sins to God at this time it was in the way of burnt offerings.