What day/time does a weekly cron start on?
I had a look though the cron man but didn’t find anything that helped 
Anyone know?
I’m giving an alternative answer here even though Trevor is correct.
The cron @weekly keyword does exactly as he mentioned. However, most distributions use run-parts to run their own scheduled crontab files (on an hourly, daily, weekly and monthly basis) which do not make use of cron’s keywords.
e.g. Ubuntu has an /etc/cron.weekly which contains a separate file for each crnojob.
This is generally defined in /etc/crontab
Ubuntu’s karmic 9.10 release has the following in /etc/crontab
17 * * * * root cd / && run-parts --report /etc/cron.hourly
25 6 * * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6 * * 7 root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6 1 * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )
So the weekly crontab in Ubuntu is run at 6.47am on Sunday
Note: when looking for manpages for crontab implementations, you want to use man 5 crontab instead of just man crontab. The latter will only give you the syntax for the crontab command. The former gives you crontab implementation details.
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