That is, can I specify for a particular file that I don't want Emacs to notify me that the file has changed on disk and prompt me about whether I want to accept changes or not? I don't want to do this globally, just for files I know will frequently be changed outside of Emacs (in particular, calendar-related Org files).
You can do this for a specific buffer with
M-x auto-revert-mode (manual). For file buffers, this will check every
auto-revert-interval seconds (defaults to 5) for changes and update the buffer from disk automatically.
Your title says "a particular file" while your question's body mentions a file type (org files). Depending on your circumstances, here are a few ways you might set this mode to get the desired effect in a persistent manner. The earlier methods are arguably cleaner and more efficient if they apply to your particular use case.
If you know you want this for every file of a certain type, consider adding a mode hook to your init setup.
(add-hook 'org-mode-hook (lambda () (auto-revert-mode 1)))
If you know you want it for everything in a certain directory, consider setting it in .dir-locals.el in the relevant location, making sure to VCS-ignore the local settings file if appropriate. This example will apply to all org-mode buffers for files under the same directory (and subdirs). You can also run
M-x add-dir-local-variable to have Emacs generate and save this for you automatically.
( (org-mode . ((auto-revert-mode . 1))) )
If neither of the two approaches above works well for you, consider a file local variable set in each file. The shortest, most general way to set one of these is to include the following in the first line of the target file:
-*- eval: (auto-revert-mode 1); -*-
In source files, the text can follow a comment delimiter (//, #, etc). You can run
M-x add-file-local-variable-propline to have Emacs generate this for you automatically, or
M-x add-file-local-variable for a format that uses the bottom of your file instead of the top.
Use this method only if you're not sharing the file with anyone else. From the GNU manual:
It is often a mistake to specify minor modes this way. Minor modes represent individual user preferences, and it may be inappropriate to impose your preferences on another user who might edit the file. If you wish to automatically enable or disable a minor mode in a situation-dependent way, it is often better to do it in a major mode hook.
Yes. Put this at the end of your file.
* COMMENT Config Local Variables: eval: (auto-revert-mode 1) End:
This assumes Org-mode, for other modes use comments in the language used as explained in the link given below.
Note: see Local Variables in Files in the manual for the background, safety information and a caveat that this might not always be a good idea. Also, the section on reverting says e.g. how to change the default, 5-secind interval for auto-reverting.