How can I group by month from a date field using Python and Pandas?

Try this:

In [6]: df['date'] = pd.to_datetime(df['date'])

In [7]: df
Out[7]:
        date  Revenue
0 2017-06-02      100
1 2017-05-23      200
2 2017-05-20      300
3 2017-06-22      400
4 2017-06-21      500



In [59]: df.groupby(df['date'].dt.strftime('%B'))['Revenue'].sum().sort_values()
Out[59]:
date
May      500
June    1000