A launchd task to run periodic-port.sh ( https://gist.github.com/2324493 )

org.macports.port.selfupdate.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>Label</key>
	<string>org.macports.port.selfupdate</string>
	<key>LowPriorityIO</key>
	<true/>
	<key>Nice</key>
	<integer>1</integer>
	<key>EnvironmentVariables</key>
	<dict>
		<key>HOME</key>
		<string>/var/root</string>
	</dict>
	<key>ProgramArguments</key>
	<array>
		<string>/usr/local/bin/periodic-port.sh</string>
	</array>
	<key>StandardErrorPath</key>
	<string>/var/log/port.out</string>
	<key>StandardOutPath</key>
	<string>/var/log/port.out</string>
	<key>StartCalendarInterval</key>
	<dict>
		<key>Hour</key>
		<integer>3</integer>
		<key>Minute</key>
		<integer>30</integer>
		<key>Weekday</key>
		<integer>6</integer>
	</dict>
</dict>
</plist>