I'd like my array to be specified based on a value elsewhere in the
spreadsheet.

e.g. I have data separated by days and months...I would like the array to be
the whole of one month's worth, so for July it might be d2:ah93 worth of data.

Normally you'd write something like =hlookup(b3,d2:ah93,5,)

However, the data to look up in b3 is a single number date e.g. 25 for 25th
of the month. I want the array to automatically search for the position in
the sheet that has the heading "July" for example, then lookup from there.
When the month changes, I want to be able to type august into my "date" box
and it return the array that starts august, so maybe ai2:bh93, instead of the
above.

Sorry this is long and complicated, any clarifications, please ask and I'll
try and help.

Thanks
Rob