Pivot tables sound like your friend. Ensure your data is in a table where each column has a heading e.g. person, Day of the Week, Percentage.
Select the whole data and then insert a pivot table, select insert on new sheet.
You will get a field selection form you want Names in rows, DotW in Columns and Percentage in data.
The right click the percentage and under the options you can change if the data presented is the average, sum, product ... Your data will be presented in a table with a row for each name, a column for each day of the week and the average of each entry for person A on day Z in the AZ cell. You will also be able to add the average for all days per person and all people per day as subtotals.
Or Manotea's average if will work, but you will have to have the list of names and days to formulate the grid of data.