Help, I need a formula for work that does the following,

In a 12 sheet excel workbook, each month of the year is represented in a
generic 31 row sheet. No data is entered in the rows representing weekends
or holidays our office is closed.

It's simple math concerning 3 cells, but the tricky part is one of the 3
cells the formula must use is in the previous working day's row. The problem
is I don't know how to tell the formula to look at the previous working day's
cell since on Monday's that would be 3 rows above and on the beginning of the
month, that would be the last cell-row with data in it on the previous
month's sheet, and then there's holidays too where rows are skipped-blank.

I.E. (formula is cell C2 =A2 -B2 +A1) In this example, Row 2 would
represent the 2nd of May. This formula works only if May 1st was a workday -
has data in it (cell A1). However if the 1st does not have data, because it
was a holiday, or weekend, then I need a smarter formula that knows to go to
the last business day's row, which in this case would be on a different sheet
in the workbook.

Thank-you for any help!