This is the 552nd most frequent Finnish word.
viimeisen
"Viimeisen" means "the last" or "of the last" in English, depending on context.
In this sentence, 'viimeisen' is in the genitive case, and it qualifies 'päivän,' indicating possession or association, meaning 'the lesson of the last day.'
Viimeisen päivän oppitunti on mielenkiintoinen.
The last day's lesson is interesting.
Here, 'viimeisen' functions in an instrumental sense, identifying someone as part of the 'last group,' contextualizing their arrival timing.
Hän oli saapunut viimeisen joukossa.
He had arrived among the last.
The word 'viimeisenä' means 'as the last' in this context, denoting the order of arrival.