Creating averages by Excel automatically knowing what number to use in the denominator

Problem.xlsx

I am trying to have Excel accurately average data by knowing how to use the correct denomintor. For instance: Columns Z and AA are totals for SBH and BH/S data for the date listed in column A. Column AB is the daily sum of these totals. I want the weekly average per person in columns AC to be figured by dividing the total in column AB by 12 (the number of columns that are providing the data) and then divided by the number of rows that have data. For example: on the dates of the 17th and 18th the daily sums are 578 and 592. I want excel to be able to know that the correct calculation should be (578+592)/12/2 because there is only data entered for 2 days. When I enter data the next day in row 23 for the 19th I would like excel to calculate in AC24 as the sum of AB21, AB22, and AB23 divided by 12 and then divided by 3 because I now have entered data for 3 days.

I also need all the monthly averages in row 36 to be calculated automatically by the number of days only for which data has been entered. For example right now on the attachment there are only 3 days so I would like Excel to realize that there is only 3 days of data and calculate the monthly averages appropriately. Because I started the sheet with data on the 14th and the 25th is a holiday there will be 10 business days of data that would be averaged.

Also I would like no data to show (including no zeros) in an cells that contain formulas.

Thank you in advance.