When is it best to post an obituary to the public platforms?  This answer varies depending on the situation.  Sometimes, a family will choose to have a private ceremony or burial and just want to announce the death of a family member.  In these cases, it falls upon the family to make the announcement when they are ready.

In cases where there will be a memorial service or funeral for the decedent, the best rule of thumb is to wait until all arrangements have been made (as soon as the time, date, place has been set).  This saves the family from having to answer  questions about when the service will be.