Prepositional phrase + punctation

On the floor ...
On friday ...
On behalf of X ...

I always thought that we need a comma after the object of the prepositional phrase, but I have seen in a lot of text where there is no comma after a prepositional phrase. When can you omit a comma?

E.g.:
On this blog I want to to show you the things that inspire me and the things I make.

I would put a comma after blog because I though that it is necessary.

This is what I would do:
On the floor, there is a spider creeping your way
On friday, I have party and a sleepover afterwards.
On behalf of John, I want to propose to you.

Can someone enlighten me? Thanks!