S3 Yom 18

The 18th day of the Biblical Hebrew month starts at:
sunset on the 20th through sunset on the 21st of the Gregorian month.

Exodus 23:12

For six days do your work, and on the seventh day keep the Sabbath; so that your ox and your donkey may have rest, together with the son of your servant and the man from a strange land living among you.