Hi guys, I have a data set spanning 2 years. I would like to average all of the values that occur in the first 1/3 (Day1-10) a second average on the second 3rd (Day 11-20) and a final average for the third 3rd (Day 21-28or30or31). Can anyone help me write the equation for such an idea?

The average ignores the month and year and just averages values in the range of days.