here is the formula:
INDEX(INDIRECT(M$8&"!BS:BS"),MATCH($B11,INDIRECT(M$8&"!B:B"),0))
the output for M$8 is week 1_mon
the formula within M$8 is "week 1_"&M10
the output for M10 = mon
I checked that the text for the name of sheet is the same as the output for M$8. The result was "TRUE".
I am perplexed!!
You need quotes round the sheet name because it has a space in it, so:
=INDEX(INDIRECT("'"&M$8&"'!BS:BS"),MATCH($B11,INDIRECT("'"&M$8&"'!B:B"),0))