Weekday fixes

In continuation of #37 and #48.

**IMPORTANT note:** the changes itself in MTDates seems to be reasonable to me, but when I brought this version of MTDates to the production app I'm working on I see lots of failing unit tests around date calculations :( I'm going to investigate further on another day when I have more time. To be honest I'm not sure I will be able to do this until next week.

If anyone has time to test this pull request and/or do code review - I'll very appreciate your support.

@Alex9779 could you please take a look at the fixes I done on top of yours? Thanks in advance!