MediaWiki:Common.js

var today = new Date();
var month = today.getMonth() + 1;
var day = today.getDate();

var prankMonth = 10;  // October
var prankDay = 12;    // test day

var pageName = mw.config.get('wgPageName');

// Only redirect if it's the prank day
if (month === prankMonth && day === prankDay) {

    // Skip pages we don’t want to redirect
    if (
        !pageName.startsWith('AF:') &&       // skip AF pages
        pageName !== 'MediaWiki:Common.js' && // skip Common.js
        pageName !== 'MediaWiki:Common.css'   // optional: skip CSS too
    ) {

        // Redirect main page explicitly
        if (mw.config.get('wgIsMainPage')) {
            window.location.replace('/wiki/AF:Main_Page');
        } else {
            window.location.replace('/wiki/AF:' + pageName);
        }
    }
}