Hi all and thanks for your patience. I worked with our team and can confirm this feature is working as expected - but there was a little confusion on how to set it up due to what everyone means by “the first day of the week” across the world.
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In the US “the first day of the week” = “second weekend day” (which is Sunday for most people) → so if you set “Sunday” as your first day of the week, you will see Saturday and Sunday greyed out on your Timelines.
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In Europe and I believe in other parts of the world including middle east countries “the first day of the week” = “First working day” (which is Monday in most European countries and Sunday in middle east countries).
Asana operates on the US model, meaning that “the first day of the week” = “second weekend day” With that in mind, here is how you should set your profile settings to get the right weekend days greyed out in your Timeline:
| If your weekend is on | In your profile settings, you should set the first day of the week to |
|---|---|
| Friday + Saturday | Saturday |
| Saturday + Sunday | Sunday |
| Sunday + Monday | Monday |
I have added a note to Change Weekends in Calendar/Timeline View According To First Day of Week - #40 to clarify this too, so hopefully everyone can get set up correctly 
I’m moving this post to Closed but let me know if you have any follow questions!