Essive case

The essive or similaris case (abbreviated ESS) is one example of a grammatical case, an inflectional morphological process by which a form is altered or marked in order to indicate its grammatical function. Marking of the essive case on a noun can express it as a definite period of time during which something happens or during which a continuous action was completed. The essive case can also denote a form as a temporary location, state of being, or character in which the subject was at a given time. The latter of these meanings is often referred to as the equivalent of the English phrase “as a ___”.

In the Finnish language, this case is marked by adding "-na/-nä" to the stem of the noun.

Use of the essive case for specifying times, days, and dates when something happens is also apparent in Finnish

In Finnish, the essive case is technically categorized as an old locative case, or a case which in some way indicates spatial location. However, in the present day language, the case has lost the majority of its spatial meaning. The case instead typically denotes a state that is temporary or inclined to change.