How do i make night and day transitions

Unity is not a "collection of scripts you attach". There are a few that come with it to do very common things, and that demonstrate how to script, but in general, you need to program things yourself. You can always search the Unify wiki for helpful scripts, but what you're talking about is not so general-purpose that I'd expect it to be there.

Here are the pages that I believe will be most helpful to you in learning what you need to learn to achieve your effect:

http://unity3d.com/support/documentation/ScriptReference/Light-intensity.html

http://unity3d.com/support/documentation/ScriptReference/Time-time.html

There are a number of ways to do this yourself, but i should point out that some very sophisticated methods have been worked out by others.

For one example is Unisky: http://www.sixtimesnothing.com/unisky which does what you want and a lot more: procedural sky, weather, and atmospheric visual effects system.

in any case, heres a stab at a simple script to do what you want:

//set times here

var day_duration:float = (your time here);

//this makes a smooth intensity transition

var phi : float = Time.time / dayduration 2 Mathf.PI; var light_amplitude : float = Mathf.Cos( phi ) * 0.5 + 0.5;

//and this will make the light change.

light.intensity = light_amplitude;

then drag that badboy onto your light. , now it changes.

if you want somethign more sexy: here is a sunlight script out there in the community. http://unifycommunity.com/wiki/index.php?title=SunLight

and your skybox? here is some very good code for fading between two skyboxes: http://www.unifycommunity.com/wiki/index.php?title=SkyboxBlended

and if you don't want to build it all alone, or straight up just buy it... you can do something in between... buy a tutorial!!

http://forum.unity3d.com/threads/47951-Video-Tutorial-Day-amp-Night-Cycle

and voila!

that should cover all your bases, sir!

 var speed : float = 0.5; 
 //this is the speed your directional light rotates at
 //adjust this to your liking
 
 function Update() {
     transform.Rotate(0,0,speed*Time.deltaTime);
 }

What this script does is rotates the directional light (NEEDS TO BE DIRECTIONAL) and when it rotates it lowers the light. Adjust to your liking. Enjoy!

PS: You can copy and paste that script into a JavaScript file and attach the JavaScript file to the directional light.