The essive or similaris case carries the meaning of a temporary state of being, often equivalent to the English "as a...".
In the Finnish language, this case is marked by adding "-na/-nä" to the genitive stem of the noun, but with strong consonant gradation. Example: "lapsi" -> "child", "lapsena" -> "as a child", "when (I) was a child".
In Finnish, it is also used for specifying times, days and dates when something happens. For example: "maanantaina" -> "on Monday", "kuudentena joulukuuta" -> "on the 6th of December". Some expressions use the essive in the ancient locative meaning, e.g. "at home" is "kotona". Observe the similarity to English "at home/in my home":
Luen lehtiä kotona. "I read newspapers at home." If you use the inessive, kodissani, you contrast to reading them in the garage (a physical location) instead.
Kodissani tehdään remonttia. "In my home, a renovation is underway."