Contrary to ISO 8601 and RFC 3339, Alertmanager regards Sunday as the first day of the week rather than Monday, so weekends cannot be written as saturday:sunday, and "every day" would have to be written as sunday:saturday rather than monday:sunday.

I think it would be even better if the "first" day of the week was an implementation detail, and ranges like friday:monday worked intuitively. I'm happy to implement this if there is agreement.