This is the 5790th most frequent Russian word.
прошлогодним
"Last year's."
In this sentence, 'прошлогоднем' is an adjective describing the suit, indicating it is from the previous year.
Он пришёл на вечеринку в прошлогоднем костюме.
He came to the party in last year's suit.
Here, 'прошлогоднем' is an adjective describing the trip, indicating it occurred the previous year.
Этот фильм напоминает мне о прошлогоднем путешествии.
This movie reminds me of last year's trip.
In this context, 'прошлогодних' is an adjective describing the elections, again indicating they took place in the previous year.