Great question. Every day can be represented as a session. Every session has an associated ID. Every time you kill a player a new entry is made with your new kill, or the previous entry is updated if you have one for this session! I'm actually doing something
similar as well for a side project! Entirely automated process as well
.
There is no announcement for the winner, however that should definitely be done, good idea. Instead the scoreboard will reflect the current winner, so it can be viewed there.