The reason we can't directly select Salesforce Contacts - from the Contact Model - is that these are not Sendable.
To bypass this we need to:
- create a Copy in a Sendable DE + Automation to update the new DE reglarly.
Notice that the only way to do this is with a SQL Query activity in Automation builder.
Select * for that SQL Query
If you try to use Filter Activity on Synced DEs, it will create another Synced DE, which is again not sendable so it doesn't solve the problem.
It is now possible to filter a DE and then modify (edit) the resulting Filtered DE to be Sendable.
- Once we create the new DE, we have to connect it to the Contact model in Contact Builder, and voilà.
The new DE is now an option under a Date-Based Event.
Hopefully this helps someone in the future. (Upvote it to let me know!)