This is the 418th most frequent Estonian word.
eelmisel
"Of the previous" or "last."
The word 'eelmisel' modifies 'aastal', specifying the year before the current year.
Eelmisel aastal õppisin uusi oskusi.
Last year, I learned new skills.
The word 'eelmisel' here is used as an adjective modifying 'nädalal', indicating that the referenced week is the one prior to the current week.
Eelmisel nädalal käisime rannas.
Last week, we went to the beach.
The word 'eelmisel' modifies 'õhtul', referring to the evening immediately preceding today.