Run Custom Function before "Save" Button is Clicked with PreSaveAction. SharePoint 2010

Add Javascript function to the page:

function PreSaveAction() 
{ 
    // your custom code goes here
    return true;  // OK to proceed with the save item }

For more information, check this post:
http://edinkapic.blogspot.ru/2007/10/add-javascript-date-validation-into.html