monthsNames.js

// Declare fallbacks
Date.monthNames = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December'];
Date.monthNamesAbbrev = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'];

$.ajax({
	url: mw.util.wikiScript('api'),
	data: $.param({ action: 'query', format: 'json', utf8: 'y', meta: 'allmessages',
		ammessages: 'january|february|march|april|may|june|july|august|september|october|november|december' +
			'|jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec',
		amlang: mw.config.get('wgContentLanguage')
	}),
	dataType: 'json',
	headers: {
		'Cache-Control': 'max-age=99999999, immutable'
	}
}).done(function(json) {
	if (!json || !json.query || !json.query.allmessages) return;
	var names = json.query.allmessages.map(function(e) { return e['*']; });
	Date.monthNames = names.slice(0, 12);
	Date.monthNamesAbbrev = names.slice(12);
});