You can do this with an array formula. In row 2, select your cell where you want the formula, paste the following into the formula bar, and press `Ctrl`+`Shift`+`Enter`.

```
=INDEX($A$1:$D$1,MIN(IF(A2:D2<0,COLUMN(A2:D2))))
```

For reference, `A1:D1`

is the range of column headers (months), and `A2:D2`

are the values in the row. What this formula does is look for values less than 0 and then takes the one with smallest column number. It then returns the month header in that column number.

This formula can be filled down to all rows.

**EDIT:**

If your data does not start at column A, you will have to make a small adjustment to the formula to get it to work. Everything stays the same in the formula except the first argument of the `INDEX`

function must be extended to column A. For example, if your data starts in column C, then you would use the following formula instead (entered as an array formula).

```
=INDEX($A$1:$F$1,MIN(IF(C2:F2<0,COLUMN(C2:F2))))
```

The reason for this is that the `INDEX`

function's second argument indicates the position in the array, not necessarily the column number. This distinction is obliterated if you just extend the array all the way to column A though (i.e., position in array will equal column number).