Run a cron job twice a week or every other week

“Twice a week” and “bi-weekly” can describe different schedules, so choose the behavior you actually need. For twice a week, the expression below runs at midnight every Monday and Thursday.

# at midnight every Monday and Thursday
0 0 * * 1,4

Read 0 0 * * 1,4 from left to right: the first 0 selects minute 0; the second 0 selects hour 0; the day-of-month * permits every date; the month * permits every month; and 1,4 selects Monday and Thursday in the day-of-week field. You may use MON,THU instead when your cron implementation supports weekday names. If you mean every other week, standard cron has no native 14-day or alternate-week field. Schedule one weekly trigger, then let the command check ISO week parity or a persisted last-run date before doing work:

# weekly trigger; the script runs the work only every other week
0 9 * * 1

The cron still fires weekly; the wrapper must skip alternate runs. In a Bash wrapper:

#!/bin/bash
# run only on even ISO weeks
week=$(date +%V); ((10#$week % 2 == 0)) || exit 0
/path/to/script.sh

ISO week 53 causes a 7- or 21-day boundary gap, so persist the last-run date for an exact 14-day cadence rather than relying on even or odd calendar weeks.