AWS Cloudwatch Cron

I was trying to schedule a once a week snapshot of a EBS volume and getting "Parameter ScheduleExpression is not valid". Turns out I missed something small. If you schedule using a cron expression note this important requirement: One of the day-of-month or day-of-week values must be a question mark (?)

I was trying:

0 1 * * SUN *

What worked was:

0 1 ? * SUN *