Hello again Yury,
I will check on the status of the “for” translation, please allow me some time to do that.
As for “previous day” and “next day”, you can add those translations to the translation files. Here’s how you can do that:
1. Grab this file: wp-content\plugins\the-events-calendar\lang\the-events-calendar-ru_RU.po and copy it to wp-content\languages\plugins\. This will allow you to keep your translations even after a plugin update.
2. Open the copied file with gettext translation editor, like poedit (you can grab it here) and add the missing translations.
3. Save the file. Make sure it creates a the-events-calendar-ru_RU.mo version in the same directory.
4. You are all set.
Let me know if this helps you out.
Cheers,
Andras
PS: If you feel like it, you can also contribute to the translation of our plugins here:
http://translations.theeventscalendar.com/projects