The firstDayOfMonth() method returns a DayPilot.Date instance representing the first day of the specified month.
DayPilot.Date.firstDayOfMonth();
DayPilot.Date representing the first day of month.
var feb1 = new DayPilot.Date("2020-02-15").firstDayOfMonth(); // new DayPilot.Date("2020-02-01T00:00:00")