df = pd.DataFrame({'A': ['loc_a'] * 12 + ['loc_b'], 'B': ['group_a'] * 7 + 
    ['group_b'] * 3 + ['group_c'] * 2 + ['group_a'], 'Date': ['2013-06-11',
    '2013-07-02', '2013-07-09', '2013-07-30', '2013-08-06', '2013-09-03',
    '2013-10-01', '2013-07-09', '2013-08-06', '2013-09-03', '2013-07-09',
    '2013-09-03', '2013-10-01'], 'Value': [22, 35, 14, 9, 4, 40, 18, 4, 2, 
    5, 1, 2, 3]})
Filling in date gaps in MultiIndex Pandas Dataframe