Would love to see this feature in your devs backlog. Other task oriented SaaS have features where you can select Weekends as None-Working Days, as well as Enter a List of Holidays as Non-Working Days.
Most of the work I am involved in is operational. We plan out this work six months to more than a year in advance - really due to the nature of our work in Examinations. Having the ability to capture these dates at an administrative level for the entire organization on Asana would be very helpful. We dont want to run into a scenario where we have planned a sprint assuming we have 5 workdays, and instead only have 4, and the sprint was not realistic. You need a good memory and there is definitely an art and skill to this.
My workaround for Holidays and other days where resources will not be available is created a Holidays and Meetings Template which gets pulled every year to create a collection of holidays and meetings (meetings where resources will be unavailable to work). When building out our operational tasks by day duration, the Holidays and Meeting Project is referenced and pulled up on a separate window. This cross reference helps ensure more time is allocated, or a task is not set or due on these dates.