Skyboxes <- [
   "brightday",
   "nightsky"
]

function ChangeTo(n)
{
    worldspawn <- Entities.FindByClassname (null, "worldspawn");
    worldspawn.__KeyValueFromString("skyname",Skyboxes[n]);
}

//so, you will need to make:
//OnTrigger, logic_script, RunScriptCode, ChangeTo(0), 0.00 - to make it birthday
//OnTrigger, logic_script, RunScriptCode, ChangeTo(1), 0.00 - to make it nightsky