When did a Celestial enter Tiz? When the party confronts Ouroboros at the Altar of Darkness when he addresses one of them he assumes their form. 


   At one point when he is talking about the Celestial Realm, he addresses Tiz and says that he sense a resident of the Celestial Realm in him.
 After Ouroboros's defeat during the credits Tiz has Egil to go on ahead before he heads to the graveyard and says:
 Tiz: I suppose i should return what i borrowed
 Before a light leaves him. Tiz collapses suggesting that the light is the Celestial leaving him.


In the teaser footage for Bravery Second unlocked after the game, when Magnolia goes to get Tiz out of where ever he is, the way she talks is as if he is talking back to her:


   Magnolia: Relax, There's no need to be suspicious of Me
 pause
 Magnolia: ...Yeh, I think that's fair to say...





   Magnolia: I envy You Tiz
 pause
 Magnolia: Because, you've got the kind of good luck i'll never enjoy...
 pause
 Magnolia: ...You get to live in the same era as me, you lucky dog, you!





   Magnolia: 'Kay, Let's get going.
 Magnolia: ...To Where?
 Magnolia: Mmm... to somewhere nice ♥ 
 So this would indicate that Tiz only had a Celestial in him, he wasn't one himself which then left his mortal form (if he was he would be unresponsive to Magnolia)


I am wondering when did a Celestial enter Tiz? and did a Celestial enter every other Tizs' in all the worlds linked by Airy or was the Tiz we play as the only one?
The celestial entered Tiz when you started a new game, then left him as the credits were ending.

You are the Celestial occupying Tiz's body.

The Celestial realm that Ouroboros is trying to break into is the real world (the 3DS camera even turns on and blends your face into the background during the final battle after Ouroboros says that he's approaching the Celestial Realm). Ouroboros is also talking directly to you when he asks if pretending to care about the party has alleviated your boredom.

The AR movie at the start is supposed to be Anges briefly breaking into the real world (the Celestial Realm) to beg you to intervene.

The extra movie is a sneak peak of Bravely Second. He wakes up from the coma because the celestial (you) came back to him for the next game.