Afmælið mitt er í næsta mánuði.

• -ið is the definite article attached to a neuter noun. afmæli = birthday; afmælið = the birthday.
• Icelandic often keeps the definite article even when a possessive is present, so afmælið mitt literally means “the birthday my,” i.e., “my birthday.”

• With a definite noun (one that has the article attached), the possessive normally comes after: afmælið mitt.
• You cannot say mitt afmælið (preposed possessive + definite suffix) — that’s ungrammatical.
• If you drop the definite suffix, you can put the possessive before: mitt afmæli. This can add emphasis/contrast: “MY birthday (as opposed to someone else’s) is next month.” Both afmælið mitt and mitt afmæli are possible, but afmælið mitt is the most neutral here.

• For months, Icelandic idiomatically uses í: í janúar, í febrúar, í næsta mánuði.
• For days of the week and many specific dates, you use á: á mánudaginn (on Monday).
• For years, you typically use á: á næsta ári (next year). So “in next month” is literally í næsta mánuði.

• í takes the dative when it means “in/inside (time/place).”
• mánuði is the dative singular of the masculine noun mánuður (month).
• næsta is the weak dative masculine singular form of the adjective næstur (next/nearest). Superlatives and “ordinal-like” adjectives (e.g., “first/next/last”) are usually weak when used before a noun, hence næsta mánuði.
• Contrast:
  • Nominative (as a subject): næsti mánuður (the next month)
  • Dative after í: í næsta mánuði

• The “prepositionless time accusative” (e.g., Næsta mánuð fer ég…) is common with action verbs.
• With the copula er (“is”), native usage strongly prefers the preposition: say Afmælið mitt er í næsta mánuði. Without í sounds odd here.

Afmælið (the-birthday) mitt (my) er (is) í (in) næsta (next) mánuði (month.DAT). So literally: “The birthday my is in next month.”

• The possessive agrees with the grammatical gender, number, and case of the noun, not with the speaker.
• afmæli is neuter singular nominative here, so you use mitt (neuter). The basic nominative forms are:
  • masculine: minn
  • feminine: mín
  • neuter: mitt

• er (“is”) states the fact that the date falls next month: Afmælið mitt er í næsta mánuði.
• verður (“will be/become”) often highlights the occurrence as an event (e.g., the celebration): Afmælið mitt verður í næsta mánuði can imply “The (party/celebration) will be next month.”

• afmæli = birthday, the fact/occasion/celebration. Very common in everyday speech for “birthday.”
• afmælisdagur = birthday day/date specifically. You can say Afmælisdagurinn minn er í næsta mánuði, but most people still just say afmæli unless they need to stress the exact date.

• í + dative = “in/during” a period: í næsta mánuði, í þessari viku.
• í + accusative can express duration: Hann var þar í mánuð (He was there for a month). So the dative in your sentence marks being “within” that time frame.

• æ sounds like “eye” (ai).
• á sounds like the “ow” in “cow,” but long and smooth.
• í is like long “ee.”
• ð (the letter eth) is a soft “th” as in “this,” especially between vowels: -ið, mánuði.
• Double consonants like -tt- in mitt are held a bit longer. A rough guide: Af-mæ-lið MITT ehr ee NAI-sta MAO-nu-thi.