Two options...
Option one:
Let F2:F13 contain your sheet names, and then use the following
formula...
=VLOOKUP(A2,INDIRECT(INDEX(F2:F13,MATCH(TRUE,COUNTIF(INDIRECT(F2:F13&"!A2
:A100"),A2)>0,0))&"!A2:B100"),2,0)
....confirmed with CONTROL+SHIFT+ENTER, not just ENTER. Adjust the
ranges (A2:A100, A2:B100, and F2:F13) accordingly.
Option two:
This doesn't require you to list your sheet names, but it uses three
cells...
B2:
=MATCH(TRUE,COUNTIF(INDIRECT(TEXT(DATE(2005,ROW(INDIRECT("1:12")),1),"mmm
")&"!A2:A100"),A2)>0,0)
....confirmed with CONTROL+SHIFT+ENTER, not just ENTER.
C2:
=INDEX(TEXT(DATE(2005,ROW(INDIRECT("1:12")),1),"mmm"),B2)
D2:
=VLOOKUP(A2,INDIRECT(C2&"!A2:B100"),2,0)
Hope this helps!