If the sheets were arranged from Jan thru Dec this would be relatively easy.

However, it appears as though the sheets are arranged based on a fiscal year order.

First thing: Make sure the sheet names all use the same naming format. Some of yours use the short name format like Apr while others use the long name format like March.

Make ALL the sheet names in one format. I prefer the short name format. So, all sheet names would be:

Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec, Jan, Feb, Mar

Create the reference table in K3:L14...

Data Range
K
L
3
Apr
50
4
May
40
5
Jun
60
6
Jul
70
7
Aug
30
8
Sep
80
9
Oct
20
10
Nov
20
11
Dec
70
12
Jan
40
13
Feb
50
14
Mar
50


Enter this formula in L3 and copy down:

=INDIRECT(K3&"!B5")

Change the source of the drop down list in cell C1. As the source use =$K$3:$K$14.

Then, the sum formula would be:

=SUM(L3:INDEX(L3:L14,MATCH(C1,K3:K14,0)))