(Just a question) How do I find the average Seasonal Index for each month?

Do I take all of the seasonal indexes from each Month-1 from each year and find the average?

Time period Month Demand Yearly average Seasonal Index Average SI Deseasonalized 1/1/17 2374 0.803157607 2/1/17 2572.5 0.87031295 3/1/17 2666.5 0.902114473 4/1/17 2978 1.007499307 5/1/17 3443 1.16481535 6/1/17 3325 1.12489429 7/1/17 1177 0.398195663 8/1/17 2519 0.852213147 9/1/17 2990 1.011559075 10/1/17 3536 1.196278559 11/1/17 3398.5 1.149760374 12/1/17 4490.5 2955.8333 1.519199341 1/1/18 2408 2/1/18 2605.5 3/1/18 2698.5 4/1/18 2965 5/1/18 3577 6/1/18 3462 7/1/18 2611 8/1/18 3027.5 9/1/18 3022.5 10/1/18 3782 11/1/18 4252.5 12/1/18 4977.5 1/1/19 2459 2/1/19 3012.5 3/1/19 3028.5 4/1/19 3405 5/1/19 3696 6/1/19 3907 7/1/19 2973 8/1/19 3285.5 9/1/19 3314.5