Find the end of the month of a Pandas DataFrame Series

You can use pandas.tseries.offsets.MonthEnd:

from pandas.tseries.offsets import MonthEnd

df['Date'] = pd.to_datetime(df['Date'], format="%Y%m") + MonthEnd(0)

The 0 in MonthEnd just specifies to roll forward to the end of the given month. Note that if we’d used MonthEnd(1), then we’d have got the next date which is at the end of the month.
If you wanted the last day of the next month, you’d then add an extra MonthEnd(1), etc. This should work for any month, so you don’t need to know the number days in the month, or anything like that. More offset information can be found in the documentation.

Example usage and output:

df = pd.DataFrame({'Date': [200104, 200508, 201002, 201602, 199912, 200611]})
df['EndOfMonth'] = pd.to_datetime(df['Date'], format="%Y%m") + MonthEnd(1)

     Date EndOfMonth
0  200104 2001-04-30
1  200508 2005-08-31
2  201002 2010-02-28
3  201602 2016-02-29
4  199912 1999-12-31
5  200611 2006-11-30

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