Help with Excel formulas

According to your description, I introduced a new column to represent a date (Month+Day/100) by a number. Then, apply the function "averageifs" to perform. However, since there are two years, I have an "if" to determine which column to be the result. You may refer to the sheet RS1 in the attachment. 

I don't like this kind of data structure. First, the column A actually representing the day in year 2018 (except 28/2/2016). Second, it is difficult to do calculation for the data. You see that there is an "if" to get the final result. It will be very complicated if there are more columns (years).

For this, I prepare the sheet RS2. In it, all the days are arranged in column A (Date Format d-mmm-yyyy). Then, applying "averageifs" to calculate the required average. 

Hope that this is helpful to you.