public bool IsDisabled { get; set; }

<DxCalendar T="DateTime" CustomDisabledDate="@OnCustomDisabledDate">
</DxCalendar>

@code {
    void OnCustomDisabledDate(CalendarCustomDisabledDateEventArgs args) {
        args.IsDisabled = args.Date < DateTime.Today.AddDays(-20) || GetDisabledDates().Exists(d => DaysEqual(d, args.Date));
    }
    bool DaysEqual(DateTime date1, DateTime date2) {
        return (date1.Year == date2.Year && date1.DayOfYear == date2.DayOfYear);
    }
    List<DateTime> GetDisabledDates() {
        DateTime baseDate = DateTime.Today;
        return new List<DateTime>() { baseDate.AddDays(-9), baseDate.AddDays(-4), baseDate.AddDays(-3), baseDate.AddDays(3), baseDate.AddDays(5), baseDate.AddDays(6), baseDate.AddDays(15) };
    }
    //...
}