Given a date, returns first day of that Week.

FirstDayofWeek.java

    /**
     * FOrces Sunday as first day of week.
     *
     * @param date given date
     * @return First day of this day's week
     */
    public static Date getFirstDayofWeek(@NotNull Date date) {
        Calendar cal = Calendar.getInstance();
        cal.setTime(date);
        int currentDay = cal.get(Calendar.DAY_OF_WEEK);
        int pastDays = Calendar.SUNDAY - currentDay;
        cal.add(Calendar.DATE, pastDays);
        return cal.getTime();
    }

    /**
     * Sunday is default first day of Week
     *
     * @param dateString String value of Date
     * @return first day of that week
     */
    public static LocalDate getLocalFirstDayWeek(@NotNull String dateString) {
        LocalDate thatDay = LocalDate.parse(dateString);
        int pastDays = thatDay.getDayOfWeek().getValue();
        return thatDay.minusDays(pastDays);
    }