+------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
|    Name    | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
+------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| Customer 1 |   1 |   1 |   0 |   1 |   1 |   0 |   0 |   0 |   1 |   1 |   0 |   1 |
| Customer 2 |   0 |   0 |   0 |   1 |   1 |   0 |   1 |   1 |   0 |   0 |   0 |   1 |
| Customer 3 |   1 |   1 |   1 |   0 |   1 |   1 |   0 |   1 |   0 |   0 |   0 |   0 |
| Customer 4 |   1 |   1 |   1 |   0 |   1 |   1 |   0 |   1 |   0 |   0 |   1 |   0 |
+------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+

Jun (or 6 as column index) for Customer 1
Sep (or 9 as column index) for Customer 2
Oct (or 10 as column index) for Customer 3, since the last 3-month occurrence begins at Oct instead of Sep
NA for Customer 4, since there's no such an occurrence