const SingleDay = 'xxx.herokuapp.com';
const DoubleDay = 'xxx.herokuapp.com';
addEventListener(
    "fetch", event => {
        let nd = new Date();
        if (nd.getDate() % 2) {
            host = SingleDay;
        } else {
            host = DoubleDay;
        }
        let url = new URL(event.request.url);
        url.hostname = host;
        let request = new Request(url, event.request);
        event.respondWith(
            fetch(request)
        )
    }
)