Learn Hardcore Finnish: Lauantai on minun lempipäiväni.

minun is the genitive form of minä (“I”), used as a possessive pronoun: minun = “of me”, “my”.
-ni is a possessive suffix meaning “my”, attached to the possessed noun: lempipäiväni = “my favourite day”.

So minun lempipäiväni literally is “my favourite-day-my”.

In standard written Finnish, the possessive suffix is the essential part, and the pronoun minun is optional and used for emphasis or clarity:

lempipäiväni – my favourite day
minun lempipäiväni – my favourite day (extra emphasis on whose it is)

In spoken Finnish, people very often do the opposite: they keep the pronoun and drop the suffix:

mun lempipäivä ≈ “my favourite day” (colloquial; mun = spoken form of minun)

So it’s not “wrong twice”; it’s “standard + emphasis”.

Can I also say Lauantai on lempipäiväni without minun?

Yes, and in standard Finnish that’s actually more typical.

Lauantai on lempipäiväni. – “Saturday is my favourite day.”

Here, possession is still very clear from the suffix -ni on lempipäiväni, so minun is not needed.

Nuance:

Lauantai on lempipäiväni. – neutral, normal statement.
Lauantai on minun lempipäiväni. – puts extra stress on minun (“Saturday is my favourite day”, possibly contrasting with someone else’s).

Both are correct and common.

Why is it minun and not minä? What case is minun?

Minun is the genitive form of minä (“I”).

Finnish uses the genitive case to mark the possessor:

minä – I (nominative)
minun – my / of me (genitive)

The pattern is:

minun lempipäiväni – my favourite day
ystäväni kirja – my friend’s book (literally: friend-my book)
opettajan auto – the teacher’s car (teacher’s car)

So, in possession constructions, the owner is in the genitive: minun, not minä.

How is lempipäiväni formed? Why is it one long word?

Lempipäiväni is a compound noun with a possessive suffix:

lempi – favourite, love (noun)
päivä – day
lempipäivä – favourite day
lempipäiväni – my favourite day (-ni = my)

Finnish loves compounds; instead of “favourite day” as two words, you get one word: lempipäivä.

Then the possessive suffix attaches to the end of the whole noun phrase (here, the whole compound):

lempipäivä
- -ni → lempipäiväni

So it must be written as one word, not lempi päiväni or lempi päivä ni.

What is the function of on here? Why do we need it?

On is the third person singular form of the verb olla (“to be”).

olla – to be
hän on – he/she is
se on – it is
lauantai on – Saturday is

In the sentence Lauantai on minun lempipäiväni, on links the subject (Lauantai) to the complement (minun lempipäiväni) just like English “is”:

Lauantai on minun lempipäiväni. – “Saturday is my favourite day.”

Unlike some languages, Finnish does not normally drop the verb “to be” in full sentences; on is required here.

Why are both Lauantai and lempipäiväni in the same form (nominative)? Shouldn’t one of them change case?

In Finnish, with the verb olla (“to be”), both the subject and the predicative (complement) are generally in the nominative case when you are simply identifying or equating things.

Lauantai – nominative singular (subject)
(minun) lempipäiväni – nominative singular (predicative)

Pattern:

Lauantai on lempipäiväni. – Saturday is my favourite day.
Tämä on talo. – This is a house.
Helsinki on pääkaupunki. – Helsinki is the capital.

So it’s normal and correct for both sides of on to be nominative in this kind of sentence.

Could I also say Minun lempipäiväni on lauantai? Does it change the meaning?

Yes, that is correct Finnish:

Minun lempipäiväni on lauantai. – “My favourite day is Saturday.”

Grammatically it is the same type of sentence: two nominative NPs linked by on.

Difference in nuance:

Lauantai on (minun) lempipäiväni.
– Focus starts with “Saturday”. You’re saying something about Saturday.
Minun lempipäiväni on lauantai.
– Focus starts with “my favourite day”. You’re telling which day fills that role.

Both are fine; Finnish word order is quite flexible, and speakers choose based on what they want to emphasize or what is already known in the conversation.

Is Lauantai always capitalized in Finnish like in English?

No. In Finnish, names of days of the week are not capitalized in the middle of a sentence.

You write:

lauantai – Saturday
sunnuntai – Sunday
maanantai – Monday, etc.

In the example sentence, Lauantai is capitalized only because it is the first word of the sentence. In the middle of a sentence you would write:

Pidän lauantaista. – I like Saturday.

So: capital letter only at the beginning of a sentence, not because it’s a weekday name.

How do you pronounce Lauantai on minun lempipäiväni?

Approximate pronunciation (with stress always on the first syllable of each word):

Lauantai – LAU-an-tai
- lau like English “laow” (a diphthong [au])
- an like “on” but with a clearer a sound
- tai like English “tie”
on – like English “on” but shorter and with a pure o.
minun – MI-nun
- mi like “me” but with a short i
- nun with u like in “put” (but a bit more rounded), both syllables short.
lempipäiväni – LEM-pi-PÄI-vä-ni
- lem: like English “lem” in “lemon”
- pi: like “pee” but short
- päi: like “pæi”, ä as in “cat”, plus i making a diphthong
- vä: again the ä like in “cat”
- ni: like “knee” but short

Each vowel is pronounced clearly; Finnish does not reduce vowels like English does.

Can lempi be used on its own to mean “favorite”, like an adjective in English?

Not in the same way as English “favorite”.

Lempi is primarily a noun meaning “love, beloved, favourite (thing/person)” in older or more poetic usage, and in modern everyday language it appears mostly in compounds:

lempiruoka – favourite food
lempiväri – favourite colour
lempipäivä – favourite day

You can’t just say *lempi elokuva as two words the way you say “favourite movie” in English; you must make a compound:

lempparielokuva (colloquial) or lempielokuva / suosikkielokuva (favourite movie)

So in normal speech/writing, think of lempi- as a prefix-like first part of a compound, not a free-standing adjective.

Is lempipäiväni singular or plural? Could it mean “my favourite days”?

In form, lempipäiväni is ambiguous between singular and plural:

singular base: lempipäivä – favourite day
plural base: lempipäivät – favourite days
possessive suffix: -ni – my

When you add -ni to the plural lempipäivät, the final -t usually disappears:

lempipäivä
- -ni → lempipäiväni (my favourite day)
lempipäivät
- -ni → lempipäiväni (my favourite days)

So lempipäiväni can mean “my favourite day” or “my favourite days”, depending on context.

In the sentence Lauantai on minun lempipäiväni, the meaning is clearly singular (“my favourite day”), because the subject is one single day: Lauantai. If you said:

Lauantai ja sunnuntai ovat lempipäiväni.
→ “Saturday and Sunday are my favourite day(s).”

Then lempipäiväni would naturally be understood as plural (“my favourite days”). Context disambiguates.

Is minun lempipäivä (without -ni) ever correct? I hear people say things like mun lempipäivä.

You’re hearing spoken/colloquial Finnish.

Standard Finnish prefers the possessive suffix:
- lempipäiväni – my favourite day
- minun lempipäiväni – my favourite day (with explicit pronoun)

Using only minun + noun without a suffix (e.g. minun lempipäivä) is considered non‑standard in formal writing, though widely understood.

Colloquial Finnish often drops the suffix and keeps only the pronoun:
- mun lempipäivä – my favourite day (very common in speech)

So:

For formal / written Finnish: Lauantai on lempipäiväni or Lauantai on minun lempipäiväni.
For informal speech: Lauantai on mun lempipäivä is what you’ll frequently hear.