» Averaging Sales Totals by Day of the Week

Problem:

Columns A & B contain dates and their matching sales totals.
We want to determine which day of the week corresponds with each date in column A, and then calculate an average sales figure for each day of the week over the whole period. That is, we want an average sales total for all of the Mondays, one for all the Tuesdays, and so on.

Solution:

Use the AVERAGE, IF, and WEEKDAY functions in the following Array formula:
{=AVERAGE(IF(WEEKDAY(A2)=WEEKDAY($A$2:$A$16),$C$2:$C$16)}