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