mysql> SELECT *
    -> FROM `test1`;
+----+-------+
| id | data  |
+----+-------+
|  1 | yes   |
|  2 | no    |
|  3 | maybe |
+----+-------+
3 rows in set (0.00 sec)

mysql> SELECT *
    -> FROM `test2`;
+-------+------------+
| fk_id | date       |
+-------+------------+
|     1 | 2007-04-02 |
|     3 | 2007-04-05 |
|     3 | 2006-01-21 |
+-------+------------+
3 rows in set (0.00 sec)

mysql> SELECT *
    -> FROM `test1`
    -> LEFT JOIN `test2` ON `test1`.id = `test2`.fk_id
    -> WHERE MONTH( date ) = '4';
+----+-------+-------+------------+
| id | data  | fk_id | date       |
+----+-------+-------+------------+
|  1 | yes   |     1 | 2007-04-02 |
|  3 | maybe |     3 | 2007-04-05 |
+----+-------+-------+------------+
2 rows in set (0.00 sec)

Wanted :

+----+-------+-------+------------+
| id | data  | fk_id | date       |
+----+-------+-------+------------+
|  1 | yes   |     1 | 2007-04-02 |
|  2 | no    |  null | null       |
|  3 | maybe |     3 | 2007-04-05 |
+----+-------+-------+------------+