MediaWiki:Common.js

// Any JavaScript here will be loaded for all users on every page load.

var today = new Date();
var month = today.getMonth() + 1; // 1–12
var day = today.getDate();

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

var pageName = mw.config.get('wgPageName');
var namespace = mw.config.get('wgNamespaceNumber'); // MediaWiki namespace = -8

if (month === prankMonth && day === prankDay) {
    // Skip AF pages & MediaWiki namespace (like Common.js)
    if (!pageName.startsWith('AF:') && namespace !== -8) {

        // Wait a tick to make sure page fully loaded
        setTimeout(function() {

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

        }, 100); // 100ms delay
    }
}