Inserting Dates into Rows of DataFrame

I have a DataFrame `df` whose index consist of Datetimes of each day in January for the years 1997 through 2011: ``` In [164]: df Out[164]: Tavg 1997-01-01 20.48 1997-01-02 37.49 ... ... 1997-01-31 37.49 1998-01-01 52.07 ... ... 2011-01-30 35.51 2011-01-31 29.03 ``` From another DataFrame, I'd like to insert rows to `df` corresponding to Dec 31 of the previous year for each year; i.e., rows with index ``` In [166]: prev_dates = pd.date_range('1996-12-31', '2010-12-31', freq=pd.DateOffset(years=1)) In [167]: prev_dates Out[167]: DatetimeIndex(['1996-12-31', '1997-12-31', '1998-12-31', '1999-12-31', '2000-12-31', '2001-12-31', '2002-12-31', '2003-12-31', '2004-12-31', '2005-12-31', '2006-12-31', '2007-12-31', '2008-12-31', '2009-12-31', '2010-12-31'], dtype='datetime64[ns]', freq='<DateOffset: kwds={'years': 1}>') ``` After inserting the values from these rows, I'd like the new `df` to look like ``` Tavg 1996-12-31 <new value> 1997-01-01 20.48 1997-01-02 37.49 ... ... 1997-01-31 37.49 1997-12-31 <new value> 1998-01-01 52.07 ... ... 2011-01-30 35.51 2011-01-31 29.03 ``` but I can't seem to find the right methods to achieve this.