fimmtudag

In a sentence

• Ég gleymi aldrei fimmtudeginum sem við héldum veisluna.

  I will never forget the Thursday when we held the party.

  Here, 'fimmtudeginum' is the definite form in dative case, expressing the specific day of Thursday during or on which an event occurred.

• Ég hitti hana á fimmtudag að fara í sund.

  I met her last Thursday to go swimming.

  In this sentence, 'fimmtudag' is used in its accusative indefinite singular form to indicate a date in the past.

• Við munum hittast aftur næsta fimmtudag.

  We will meet again next Thursday.

  Here, 'fimmtudag' is being used in its accusative indefinite form to indicate a specific upcoming date.