This is the 3rd most frequent Danish word.
en
"En" in Danish means "a" or "one" (used as an article or numeral).
Here, "en" is the indefinite article for 'hammer,' which is a common gender noun in Danish.
Han har en hammer, der er meget stor.
He has a hammer that is very big.
In this sentence, "en" functions as the numeral 'one,' specifying a single day.
En dag vil jeg rejse til Danmark.
One day, I will travel to Denmark.
Here, "en" is the indefinite article signifying a single masculine or common gender noun, 'dog'.