Gjalddaginn er á mánudag, svo fyrsta afborgunin þarf að koma fyrir helgi.

Days of the week are normally not capitalized in Icelandic, so mánudag is written in lowercase unless it starts a sentence.

The phrase á mánudag means on Monday. With days of the week, Icelandic commonly uses á plus the appropriate case form, and here that form is mánudag. In simple schedule-style statements, Icelandic often leaves off the definite article, so á mánudag sounds natural.

Afborgun means installment, and afborgunin means the installment. The adjective fyrsta means first and agrees with the noun.

Because the noun is definite, the adjective follows the definite pattern too. In this particular form, fyrsta happens to look the same as it would in an indefinite phrase, so the grammar is there even though you do not see a different ending.

The dictionary form is þurfa, meaning to need or to have to. In the sentence, the subject is fyrsta afborgunin, which is singular, so the verb must also be singular: þarf.

So:

• þurfa = infinitive
• þarf = 3rd person singular present

Icelandic often uses koma for things like payments, letters, emails, or money arriving. So afborgunin þarf að koma literally means the installment needs to come, but idiomatically it means the installment needs to arrive or be received.

A more formal verb you may also see in this kind of context is berast.

Here fyrir means before in a time expression, so fyrir helgi means before the weekend.

Icelandic often leaves out the article in short, practical time expressions like this, especially in scheduling or deadline language. So fyrir helgi sounds natural and idiomatic. A form with the article can exist in some contexts, but this version is the most straightforward here.