Unfortunately, operational hours have always excluded weekends going back to version 9.0 and earlier. The help text in Admin has not changed from the original release:
"Operational hours define the hours, between Monday and Friday, over which service-level agreements (SLAs) are enforced. For example, if an SLA requires a response within 4 hours, and the operational hours are set from 8:00 AM to 6:00 PM, the time between 6:00 PM and 8:00 AM of the next business day does not count towards this total. Days that have been configured as days off will not count towards operational hours. However, individual SLAs can be configured to ignore operational hours.
In addition, operational hours are used to calculate the end dates of ticket tasks. When a ticket task becomes active, its end date is calculated by adding the specified number of "Complete Within" hours, treated as operational hours, to the start date.
If no start or end times are specified for operational hours, all of Monday through Friday, from 12:00 AM to 11:59 PM, will be considered operational hours. However, days off will still not be treated as operational time."
You can choose to use or not use Operational Hours at an SLA-level but that is not possible for ticket tasks. At the point that you would enable Operational Hours for a ticketing application, it will always use Mon-Fri for ticket task calculations.
Did you perhaps recently enable Operational Hours in a ticketing application? You could turn this behavior off by clearing out the Operational start and end times. The drawback to that approach is that you won't get the benefit of operational hours for other areas like SLAs.