Currently I am trying to learn Python. Searching for relevant material I came across
and books with titles such as Learn Python in one day, in 24 hours, etc.
Similar material exists for other programming languages. Of course you cannot learn (starting from zero) a programming language in such a limited time, but still you may grasp the basics.
However a quick "googling" reveals that it is not an easy task to find something relevant for the Mathematica language (i.e. Wolfram language). E.g. Wolfram Language Fundamentals from M. Gaylord comes very close.
So consider the scenario that someone wants to grasp the basics of the Wolfram language or someone wants to present the basics of Wolfram language. We have one hour or one day, it depends:-)! We may even want to write a 20 pages hands-on document.
Which material is considered absolutely indispensable?