This is the 227th most frequent Estonian word.
päev
"Day"
In this sentence, "päev" is used as a noun to indicate the time unit of a day.
Täna on ilus päev.
Today is a beautiful day.
In this sentence, "päeva" is the genitive case of "päev," specifying a certain day being referred to.
Ootan väga seda päeva, kui sa tagasi tuled.
I am greatly looking forward to the day you come back.
Here, "päeval" is the adessive case of "päev," indicating a specific timeframe during the day.