def leapYear(year):
    """Most common first.

    >>> leapYear(1956)
    True
    >>> leapYear(1957)
    False
    >>> leapYear(1900) # No leapday on century crossings.
    False
    >>> leapYear(2000) # But there is a leapday on millennium crossings.
    True
    """
    if year % 4 != 0:
        return False
    if year % 400 == 0:
        return True
    if year % 100 == 0:
        return False
    return True
