To find the first occurrence of a value in a row of cumulative sales data, you can use the MATCH function in Microsoft Excel. Here's how:

1. Select the cell where you want to display the first occurrence of the value.
2. Type the following formula: =MATCH(0,A1:A10,0)
   (Note: Replace "A1:A10" with the range of cells that contains your cumulative sales data.)
3. Press Enter.

The MATCH function will search for the first occurrence of the value "0" in the range of cells you specified. This value represents the first month where there were no sales for the item. If there were sales every month, the function will return an error.

You can modify the formula to search for a different value by replacing "0" with the value you want to find. For example, if you want to find the first occurrence of the value "100" in the range A1:A10, you would use the formula: =MATCH(100,A1:A10,0)

Once you have the formula in place, you can copy it to other cells in the same column to find the first occurrence of the value in other rows of data.