Hi @Izobelle 
We still do not have an ETA to provide on when this ability will be available, something you can do in the meantime to voice your support of this addition would be to vote for this feature in the Feedback Section of our Community and even comment showing your own use case and why it would be important to add!
Here is a post I found that you can vote for in the meantime: Vote Holiday/Blackout days in Timeline/Gantt Views
These are taken into consideration when we look at changes and additions to make to the platform, so thank you all for sharing your thoughts and feedback!
Cheers,
Jenna