I can't seem to find an event for it but try doing it for an if statement? Ofcourse it depends on what you use it for.. if(getServer().getWorld(WORLDNAME)getTime()) > TIMENIGHT) { Do blablabal... }
In a loop or something because it constantly needs to check for it... but it'll take a lot more processes I think.. It's not advised, but I really don't know where to put it..
Check this out: http://wiki.bukkit.org/Scheduler_Programming Make a repeating task that goes every 10 seconds or so, and check the time, do what you need to do.