While we do not yet support the date specs for CalendarEvent the left
out "weekly" special expression[0] dies not requires that support.
It is specified to be equivalent with `Mon *-*-* 00:00:00` [0] and
this can be implemented with the weekday and time support we already
have.
[0]: https://www.freedesktop.org/software/systemd/man/systemd.time.html#Calendar%20Events
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>