where??? I'd like it for my Leafeon ^_^
TM11 (Sunny Day) is on Route 212 to the west of the Pokemon Mansion. You will need Cut to access it.
Additionally, you can exchange 10 Red Shards for TM11 at the shard collector's house in the southern section of the route. This is repeatable.
route 212.
http://www.serebii.net/diamondpearl/tmhm.shtml
for all the tms in diamond/pearl