“Nippupäivä” is essentially a mock defence – the seminar where the first manuscript version of a thesis (or dissertation) can be discussed in its entirety for the first time – having previously only been considered in its various constituent parts. “Nippu” is literally a big pile of papers, so the idea is that this is the day when you gather all your work together properly for the first time. The purpose of the seminar is to discuss the thesis with (usually two) reviewers, or opponents, who are invited there from outside the writer’s own seminar or research community. In this defence, the first reviewer is also not a tutor.
Opponents read and comment on the manuscript from their “outside” perspective during this defence, dealing with the various parts (theoretical framework, methodology, and so on) and how these relate to the work as a whole. This is to assess just how “watertight” the arguments in the thesis will be outside the writer’s own research community. This is an important part of finalising the writing and research process, after which the thesis is ready for the formal preliminary examination.
If you have any concerns or questions about your nippupäivä, you should check these over with your supervisor.