The base
keyword can do the trick (see docs):
s.resample('24h', base=5)
Eg:
In [35]: idx = pd.date_range('2012-01-01 00:00:00', freq='5min', periods=24*12*3)
In [36]: s = pd.Series(np.arange(len(idx)), index=idx)
In [38]: s.resample('24h', base=5)
Out[38]:
2011-12-31 05:00:00 29.5
2012-01-01 05:00:00 203.5
2012-01-02 05:00:00 491.5
2012-01-03 05:00:00 749.5
Freq: 24H, dtype: float64