When to say it's a missed period - - means mine is usually 24 days cycle, so after 24 its missed or by calendar date wise say it was on 23rd last month and after crossing 23rd this month is a missed period?

The way I remember it, when my period came on the dot, the same time every month, without fail and when it didn't, well, it was a missed period and I was pregnant.

Tick

Its late once 24 days has passed (in your case). Its not by the calendar date but by how many days have passed compared to your typical cycle.

That being said, 24 days is on the shorter side (most are 28-31) and periods can be a few days late for many reasons.