Here are two common ways to calculate the sum based on weekdays or weekends in Excel:

Method 1: Using SUMIFS and WEEKDAY functions (works for Excel 2007 and later)

1. Identify your data range: Determine the range of cells containing the values you want to sum. Let's say your data is in cells A1:A20.
2. Weekday criteria: Decide which numeric value represents weekdays (typically 1 to 5) and weekends (typically 6 or 7). Weekday functions can vary slightly depending on your Excel version, so double-check the specific output for weekdays and weekends.
3. Create separate formulas: You'll need two formulas, one for weekdays and one for weekends.

* Weekday Sum: In a separate cell (e.g., B1), enter the formula:
`=SUMIFS(A1:A20, WEEKDAY(A1:A20,2)<=5, A1:A20)`
- This formula uses SUMIFS to sum the values in A1:A20 (replace with your actual range) where the corresponding weekday number in the same range (WEEKDAY(A1:A20,2)) is less than or equal to 5 (weekdays).

* Weekend Sum: In another cell (e.g., B2), enter the formula:
`=SUMIFS(A1:A20, WEEKDAY(A1:A20,2)>5, A1:A20)`
- This formula uses SUMIFS again to sum the values in A1:A20 where the corresponding weekday number is greater than 5 (weekends).

Method 2: Using SUMPRODUCT and WEEKDAY functions (more versatile)

1. Identify your data range: Similar to method 1, determine the range of cells containing the values (e.g., A1:A20).
2. Weekday criteria: Decide on the weekday and weekend numeric representations.
3. Weekday Sum: In a separate cell (e.g., B1), enter the formula:
`=SUMPRODUCT((WEEKDAY(A1:A20,2)<=5)*A1:A20)`
- This formula uses SUMPRODUCT to multiply an array of TRUE/FALSE values (based on weekdays <= 5) with the corresponding data values in your range. TRUE values (weekdays) are then summed along with their corresponding data.

* Weekend Sum: In another cell (e.g., B2), enter the formula:
`=SUMPRODUCT((WEEKDAY(A1:A20,2)>5)*A1:A20)`
- This formula uses SUMPRODUCT similarly to the weekday sum, but checks for weekdays greater than 5 (weekends).

Both methods achieve the same result of separate sums for weekdays and weekends. Choose the method that best suits your comfort level or specific needs.