def get_firstday_of_month(date):
    """''
    date format = "YYYY-MM-DD"
    """

    year, month, day = date.split("-")
    year, month, day = int(year), int(month), int(day)

    days = "01"
    if int(month) < 10:
        month = "0" + str(int(month))
    arr = (year, month, days)
    return "-".join("%s" % i for i in arr)