opportunity how to show the close date show msg close date in near throw on vf page

Question

shows the message(Close date is near) on opportunity when a particular opportunity's close date is 5 days ahead

Answer 1

In addition to the Lightning Experience suggestion from @Raspi, for Salesforce Classic, you can simply create an image formula field that looks like this:

IF (TODAY() + 5 = CloseDate,
  IMAGE("/img/samples/flag_yellow.gif" , 'Yellow' ,12,12) & ' Close Date is imminent. Do something!' & BR(),
  NULL) 

and place the formula field on the page layout, perhaps in an untitled section at the top. I use this technique to display 'soft alerts' that appear only when viewing a record. You can have many image expressions within one formula field.

Answer 2

Do you need it in the current Record Page as 'advise' ?

I usually use a Lightning component with some rules, using TOAST advises. An invisible Lightning component that handler the INIT and if some rules are true, then i call 'showToast' function:

showToast : function(component, event, helper) {
var toastEvent = $A.get("e.force:showToast");
toastEvent.setParams({
    "title": "Close Date is near!",
    "message": "The Close date is 5 days ahead!",
    "type": "warning",
    "mode": "pester"
});
toastEvent.fire();}

See documentation here: Lightning Component Show Toast

IMPORTANT! : Just for Lightning Experience Layout. If do you need for Salesforce Classic, you must build a Visualforce Page with an 'alert()'.