How can I label an event happening "every Monday" using HTML5 microdata

How can I label an event happening "every Monday" using HTML5 microdata

I'm trying to use HTML5 microdata to label events in my website. The
problem is that my events have a weekday frequency (for example, "Every
Monday from 20:00 to 22:00 hs") and according to the HTML5 microdata
specification start-time should be specified using the ISO 8601 date
format but I cannot figure out how to specify "Every Monday" using ISO
8601.
Is it possible to do that? Is there an alternative to label weekday events?
Thank in advance.