There's more to it than this, but you're probably looking for this list:
B business day frequency
C custom business day frequency (experimental)
D calendar day frequency
W weekly frequency
M month end frequency
BM business month end frequency
MS month start frequency
BMS business month start frequency
Q quarter end frequency
BQ business quarter endfrequency
QS quarter start frequency
BQS business quarter start frequency
A year end frequency
BA business year end frequency
AS year start frequency
BAS business year start frequency
H hourly frequency
T minutely frequency
S secondly frequency
L milliseconds
U microseconds
Source: http://pandas.pydata.org/pandas-docs/stable/timeseries.html#offset-aliases