Can I send evaluation requests manually and not on predefined dates?

Yes, in order to do it, click on the Events in the top menu -> find the event you would like to send an evaluation request for -> find the Evaluations tile and click View -> in the appearing window, click Send evaluation request. You will see a window where you can edit the text of the evaluation request and then send it.

Note: If you have the Automated Follow-Up Emails option enabled, those attendees who have not submitted an evaluation yet, will still receive automated follow-up emails on predefined dates.