Learn Icelandic: Veislan er á föstudaginn.

In Icelandic, when we talk about an event happening on a specific upcoming day (like this coming Friday), we use the preposition á followed by the accusative case. If the party happened regularly every Friday, we would use the dative case instead.

Why does föstudaginn have the definite article -inn attached if the English translation just says 'Friday'?

Icelandic requires the attached definite article (the 'the' ending) when talking about a specific upcoming day of the week. So while in English we just say 'on Friday', Icelandic literally says 'on the Friday'.

How do we get from the dictionary word föstudagur to föstudaginn?

First, the preposition á for a specific upcoming day forces the word into the accusative case, dropping the -ur ending to become föstudag. Then, we add the masculine accusative definite article -inn to the end, giving us föstudaginn.

Why does 'the party' end in -n (veislan) while 'the Friday' ends in -inn (föstudaginn)?

This is because of noun genders. The word for party, veisla, is a feminine noun, and its attached 'the' ending is just -n. Days of the week are masculine nouns, so they take masculine endings like -inn.