This is the 615th most frequent Czech word.
lednu
"Lednu" is the locative or dative singular form of "leden," meaning "January" in Czech.
Here, 'lednu' is the form of the noun used with prepositions to indicate 'in January'.
Narodil jsem se v lednu.
I was born in January.
In this example, 'lednu' is used to express a temporal location during the month.
V lednu bývá často sníh.
In January, there is often snow.
In this case, 'lednu' is part of a phrase expressing a deadline.