from datetime import datetime


def get_solstice_type(date):
    """Returns a string representing the type of solstice based on what month
    the solstice occurs on. It is assumed the date being passed has been
    confirmed to be a solstice.

    Keyword arguments:
    date -- a YYYY-MM-DD string.
    """

    month = datetime.strptime(date, '%Y-%m-%d').month

    if month == 6:
        return 'june'
    elif month == 12:
        return 'december'
    else:
        return None