Dasara or Vijaydashami is the day when Lord Rama defeated and killed Ravana. It is the day celebrated throughout India and is considered the vindication of truth over untruth.
If you want to know how to say Happy Dasara in Malayalam, this is the phrase that you can use.
Happy Dasara in Malayalam
Happy Dasara
ദസറ ആശംസകള്
Dasara Aashamsakal