I have long animation. And at frame 5000 I want my day world to be shown. Is it possible to combine nights and days worlds and switch between them at certain frame or I have to use 2 different blend files for that and then combine animations in video editor?
Is it an instant change or a dissolve? How are your day and night worlds set up-- By scene, by layer…?
For example i will be using multiple cameras. So lets say at frame 1 Camera 1 recording days views then this camera finishes to record at frame 200 and at this frame 200 my second camera will be recording something in a different place during the night . So yes instantly at frame 200 my camera 2 will see night(or day doesnt matter). For the dissolve i would probably animate background strenght. For the instant change you would also can say just animate background node to 0 instantly. But i have not the same node setup for the night. Night world setup is with the stars…
Hmm. Maybe there is the way to put them both together in one node group and animate mix node from 1000 (day) to 0 (night). What do you think?
Bzzzt! Nope! “Welcome to the world of film-making!” 
The production process of any film, whether that film is “real-world” or “CG” or some combination of the two, consists of two distinct parts: (1) “shooting” the film, and then (2) “editing” it.
Your movie appears now to consist of at least two major shots – “day,” and “night” – each one of which might have been captured by two-or-more cameras.
Therefore, “in step (2),” these four-or-more “pieces of film” will become: “footage,” to be (somehow) stitched-together into a final movie.
“Now, here’s the trick …” “OpenGL Preview” Renders!
Having set-up each shot, you can use this facility to render geometrically-accurate versions quickly. You can use these quickly-produced “strips of film” to final(!)-cut your project.
Since these previews are perfectly-accurate except for visual detail, you can use them. After you have thereby determined exactly what frames are needed, and from what “strips,” you can one-by-one render exactly those finished-strips, and drop them one-by-one into your project.
Personally, I find that the best workflow is to be “positively wasteful(!)” with ‘film!’ Set up a scene, set up cameras, and then, “just start shooting!” (Go ahead, conjure your very best Waterworld.) (OpenGL Preview footage can be functionally regarded as “free.”) Be sure to label each shot with the frame-number, camera-name and filename. “If you think that you might use it, shoot it!”
Then, take-off your “CG” hat and put on your “film editor” hat. The field-project delivered “miles of film,” and now it’s your job to “make sense of it” … b-u-t … the footage really didn’t cost anything to produce, so there will be no “studio bosses breathing down your neck” other than yourself. 
Spend the next however-many days hammering “all that footage” into “a movie.”
Then, switch back to “CG mode.” Once you’ve decided upon your “shot list,” shot-by-shot, camera-by-camera, and frame-by-frame, you know how to apportion your CPU-resources to the task of executing each shot in the most-efficient way.