Don't know why yours would run at midnight, but here:
# The daily script removes old log files, "scratch" and "junk" files, backs-up the NetInfo database (Mac OS X 10.4 Tiger® and earlier), reports a variety of system and network statistics, and rotates the system.log file. Under Tiger, the daily script also cleans up scratch fax files and prunes asl.log, the log file for the then-new Apple System Logging facility. Under Mac OS X 10.5 Leopard®, the daily script also prunes the asl.db file that replaced the asl.log file for Apple System Logging.
# The output from the daily script is written to the /var/log/daily.out file, which can be viewed in Console.
# By default, the daily script is scheduled to run daily at 03:15 hours local time.