### Steps to reproduce

1. Login as a user with permissions to manage work packages

2. Go to a work package page

3. Click on a date field to enter start date or finish date

4. Uncheck "Working days only" 


### What is the buggy behavior?

* Non-working days (such as weekend days) cannot be selected Begin of the insertionuntil at least one day is selected on the calendarEnd of the insertion

<img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/339752/content">


### What is the expected behavior?

1. We should be able to select them Begin of the insertionimmediatelyEnd of the insertion


### Environment information

**OpenProject version**

Core build

[c6eaf03d030db95187117e1e754c219d7d8a9ad1](https://github.com/opf/openproject/commits/c6eaf03d030db95187117e1e754c219d7d8a9ad1)

Product version

[79e94fc8ce0c56340ccde1cb1ab146313456604e](https://github.com/opf/saas-openproject/commits/79e94fc8ce0c56340ccde1cb1ab146313456604e)

Core version

OpenProject 15.4.0

**Browser**

* [x] Chrome
* [ ] Firefox
* [ ] Safari
* [ ] Mobile Safari
* [ ] Other (please specify)

**Operating System**

* [ ] Windows
* [ ] Mac OS X
* [ ] Mobile iOS
* [x] Mobile Android
* [x] Linux (please specify distro)
* [ ] Chrome OS
* [ ] Other (please specify)