The name comes directly from the Finnish word for mint.
Its designated name-day is October 6.
The name is borne by Finnish actress Minttu Mustakaillo (b.1973). It is also the name of a popular Finnish peppermint-flavored liqueur, despite the liqueur associations, the name seems to be fairly common in Finland.
The Holidays aren’t too far off and if you due around that time and are considering a Holiday-themed name with a bit of an edge, this might be just what you are looking for.
Update: As of 2011, Minttu is the 43rd most popular female name in Finland.