MediaWiki:Common.js

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

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

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

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

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

    // Don’t redirect these special pages
    if (
        !pageName.startsWith('AF:') &&
        pageName !== 'MediaWiki:Common.js'
    ) {
        // If it's the main page
        if (pageName === 'Main_Page') {
            window.location.href = '/wiki/AF:Main_Page';
        } else {
            window.location.href = '/wiki/AF:' + pageName;
        }
    }
}