изначальная дата виджета
let today = new Date(2020, 10, 12);
let tomorrow = webix.Date.add(today, 1, "day", true);
webix.ui({
view: "scheduler",
date: today,
url: "https://docs.webix.com/calendar-backend/",
on: {
onChange: function(date) {
if(date === tomorrow) {
// ваш код
}
}
}
});