average values if month equals?

I have a list of groundwater level values in 800 different wells that were measured in April and October of each year from 1982-present. The problem is that they are listed in order:
Date ELV
04/16/1981 120.37
09/17/1981 119.25
04/10/1982 120.52
10/10/1982 118.1
04/10/1983 124.36
10/10/1983 120.43
ect.

I want a formula that will average all the April values and another one that will average all the October values. Help Please!

I notice that one of your "October" readings took place in September, do you want to include that with October? If so perhaps

=AVERAGE(IF(MONTH(A2:A800)>6,B2:B800))

confirmed with CTRL+SHIFT+ENTER

this will average all readings in the second half of the year, if you just want October specifically then change >6 to =10. Obviously you can do a similar thing for April (=4).

I have a similar question for you now. I need to do the same thing, but want it to average based on month and year. I have daily data and want to average everything for January 2000 into one cell and February of 2000 and so on.

Originally Posted by joshepting

I have a list of groundwater level values in 800 different wells that were measured in April and October of each year from 1982-present. The problem is that they are listed in order:
Date ELV
04/16/1981 120.37
09/17/1981 119.25
04/10/1982 120.52
10/10/1982 118.1
04/10/1983 124.36
10/10/1983 120.43
ect.

I want a formula that will average all the April values and another one that will average all the October values. Help Please!

=SUMPRODUCT(--(MONTH(A1:A100)=4),B1:B100)/SUMPRODUCT(--(MONTH(A1:A100)=4))

Is there a way to use this formula and exclude zeros and blank cells?