Multiplayer? Won't work as easy as that.
If it's possible to trigger a light_enviroment on/off - you can fake the lighting levels by using a bunch of those. The sky can't be changed ingame though. You'll have to go around that and either use one sky that would work both with day and night, or replace the skybox with a func_wall, with it's render mode set to color and make it fade with the light_enviroment from blue to black, or however you want it.