I will try to give the information which I have This may not
sound very Convincing But it sure a very good theory.
Raghukalam, YamaGnadam , Kulikai (another one is also there)
These are the time when the planetary positions are not
favarouble (Have a bad effect on the Biorythms and thus
affecting the happenings . Thus it better to avoid.
(do you know there some Deities who are worshipped only in
raghukalam or similar? I think One is Iddumban (a demigod)
who is a devvotee of Lord Muruga or Karthikeyan)