Replace welcome screen (aka "splash" screen) with calendar?

If you use org-agenda you can try the extension Emacs Dashboard. https://github.com/rakanalh/emacs-dashboard

I hope so ...

• You should append into ~/.emacs file, as a first line, next command:(setq inhibit-splash-screen t) ;; it'll disable splash screen
• At the end of ~/.emacs join the next sequence:

  (let ((buffer-cal "*Calendar*"))
    ;;
    ;; next commands:
    ;;
    ;; create calendar and pass focus to it
    ;;
    (and
     (calendar) (bufferp (get-buffer buffer-cal))
     (switch-to-buffer buffer-cal)
     )
    (delete-other-windows)
  )
  

You can customize option initial-buffer-choice (using M-x customize-option).

The value can be a buffer name (the name of your calendar buffer, if it exists - e.g. "*Calendar*") or a function that returns a buffer. The buffer is displayed in place of the welcome (aka "splash") screen.

It is of course up to you to ensure that when you start up Emacs the necessary function has been defined or the named buffer has been created.

See the Emacs manual, node Entering Emacs.