May be this tool can help you: crontab.guru
if I understand your question is something like this:
#+--------------------------- Minute (0-59)
#| +---------------------- Hour (0-23)
#| | +---------------- Day (1-31)
#| | | +------------ Month (1-12)
#| | | | +-------- Day of week (0-6, 0=Sunday)
#| | | | | +--- Command to be run
#| | | | | |
#v v v v v v
#====================================================================
# run task At 00:00 on Monday.
0 0 * * 1 run_task.sh
In run_task.sh
you can put the condition.
# script to check if the task run 20 days ago.
aux_file="/tmp/auxdate.txt"
# this compare the actual date with the date in the file
# if the diference is >= 20 the task run
if [[ ! -e "$aux_file" || $(( ($(date '+%s') - $(cat auxdate.txt))/(60*60*24) )) -ge 20 ]]; then
echo $(date '+%s') > "$aux_file"
echo "RUN TASK"
# run task...
else
echo "NOT RUN TASK"
fi