Fix bug in calculation of holidays
Closes #31415

- [x] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @dhirschfeld! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-02-07 04:22:19 UTC Can you add tests for the behavior this fixes CI is passing! :tada: code change looks fine. I think you need to isort the files you change. btw we have a pre-commit hook for this (pip install precommit turns it on).

merge master and ping on green. I rebased on master but I think I'm now getting an unrelated failure bout `libindex`:
```
black --version
black, version 19.10b0
Checking black formatting
All done! ✨ 🍰 ✨
941 files would be left unchanged.
Checking black formatting DONE
flake8 --version
3.7.9 (flake8-comprehensions: 3.2.2, mccabe: 0.6.1, pycodestyle: 2.5.0, pyflakes: 2.1.1) CPython 3.7.6 on Linux
Linting .py code
##[error]./pandas/core/series.py:1025:9:F821:undefined name 'libindex'
##[error]./pandas/core/series.py:1108:17:F821:undefined name 'libindex'
Linting .py code DONE
``` Possibly related to #31506? ping @jreback - rebased on (the latest) master and the tests are passing! :tada: ping @jreback!  thanks @dhirschfeld very nice