Figure out how many rows of data (daily stock returns) you have.
In an adjacent column starting with the same row as your data, enter the numbers 1 and 2. Move the cursor over the right-bottom cell containing the 2 and double-click. Each row should now be sequentially numbered.
Divide the total number of rows by 6. Say this result is N. Identify the rows containing N, N*2, N*3, etc. in the column added above. These are the markers for the desired ranges.