When should I use Prep "on" before a week date?

[Yxr]

For example:

The inquest began Monday with the driver taking the stand.

Why there is no "on" before "Monday" ?
Under what situation I don't need preposition?

[Cerberus™]

I suppose it depends on whom you ask: some call dropping the preposition natural, others sloppy. If you do it, I don't think it matters in which situation you do it, the exceptions being "last Sunday" and "next Monday", which are acceptable to most, if not all.

[Yxr]

Thanks