This feature is uses numbered days of the month for watering instead of specific days of the week (Odd days 1st, 3rd, 5th, etc.; Even days 2nd, 4th, 6th, etc.).
When in the Water Days mode, press the button until the cursor is above either ODD or EVEN on the display.
Press the button to select, or the button to cancel either ODD or EVEN days to water.