As LSletty said, if you want to know when it is called, use the handler from .load () itself:
$("#MainContent").load("path/content.html", function(){
More info here: jQuery download event
To complete the action after completion, I would use the .done () handler.
Use it as follows:
$("#MainContent").load("path/content.html").done(function(){
From jQuery docs:
Add handlers that will be called when the Deferred object is enabled.
More info here: deferred.done () jQuery
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