» Finding the First Value in a Row and Retrieving the Header for That Column

Problem:

Columns B:G contain the monthly payments for the first half of the year that were made for each investment in column A.
The payments for each investment may start or end on any of the months.
We want to retrieve the month on which payments for each investment started.
Solution:

Use the INDEX, MIN, IF, and COLUMN functions as shown in the following Array formula:
{=INDEX($B$1:$G$1,MIN(IF(B2:G2<>"",COLUMN($B$1:$G$1)-COLUMN(B1)+1)))}