USMemorialDay defined incorrectly.

[zegres]

Memorial day is incorrectly defined.

>>> from pandas.tseries.holiday import AbstractHolidayCalendar, USMemorialDay
>>> class MemorialDayCalendar(AbstractHolidayCalendar): rules=[USMemorialDay]
>>> MemorialDayCalendar().holidays('2021','2022')
<class 'pandas.tseries.index.DatetimeIndex'>
[2021-05-24]
Length: 1, Freq: None, Timezone: None

The actual date of Memorial day in 2021 is 5/31.

Memorial day should be defined as:
Holiday('Memorial Day' , month=5 , day=25, offset=DateOffset(weekday=MO(1)))

[rockg]

One would think the test spanning 10 years was enough. I agree that this is right...care to do a Pull Request?

[zegres]

OK, I made a pull request.

[jreback]

closed by #10282