Run cron job every other week.

Run cron job every other week, let’s say Friday at 10AM. Add this to your crontab:

0 10 * * 5 [ `expr \`date +\%s\` / 86400 \% 2` -eq 1 ] &&

Every second Friday is an odd number of days since Thursday, Jan 1, 1970 and every first Friday is an even number, plus there are 86400 seconds in a day.
If there is “-eq 1” this means it will not be executed this Friday only every other Friday, if there is “-eq 0” this means it will executed this Friday and every other Friday.