Lauantai-iltana olen pitkään hereillä.

In Finnish, lauantai-ilta is a compound noun: lauantai (Saturday) + ilta (evening) → lauantai-ilta (Saturday evening).

Spelling rules:

• Most compounds are written as one word.
• But when the first part ends in the same vowel that starts the second part, a hyphen is usually written to make reading easier:
  • lauantai-ilta (ai + i)
  • maa-alue (aa + a)

So:

• lauantai-ilta (base form)
• In the sentence we need the essive case (more on that below), so it becomes lauantai-iltana.

lauantai iltana (two separate words) would normally be understood as:

• lauantai (Saturday, basic form, no case) + iltana (in the evening),
  which is not how native speakers express on Saturday evening. The natural way is with the compound: lauantai-iltana.

The ending -na / -nä is the essive case. One of its common uses is in time expressions like on [a particular] day/part of the day.

Patterns:

• With just a part of the day, Finnish typically uses adessive:
  • illalla = in the evening, at nightfall
  • aamulla = in the morning
• But with day-of-week + part of day, Finnish typically uses the essive on the whole compound:
  • maanantai-aamuna = on Monday morning
  • sunnuntai-iltana = on Sunday evening
  • lauantai-iltana = on Saturday evening

So lauantai-iltana is the normal, idiomatic way to say on Saturday evening.

You can say lauantaina illalla, but it sounds a bit heavier and more like “on Saturday, in the evening” (two separate time phrases), rather than the compact idea on Saturday evening as one unit.

lauantai-iltana in the singular essive usually refers to one particular Saturday evening, either:

• a specific coming/previous one:
  • Lauantai-iltana olen pitkään hereillä.
    = This Saturday evening I’m awake for a long time.
• or a specific one understood from context.

If you want to say on Saturday evenings (in general, habitually), you would normally use lauantai-iltaisin:

• Lauantai-iltaisin olen pitkään hereillä.
  = On Saturday evenings, I’m (usually) awake for a long time.

So:

• lauantai-iltana → one concrete Saturday evening (or at least treated as a single occurrence)
• lauantai-iltaisin → every/most Saturday evenings, a repeated habit

pitkään is an adverb derived from the adjective pitkä (long / tall).

• pitkä = an adjective, used with nouns:
  • pitkä matka = a long trip
  • pitkä päivä = a long day
• pitkään = used like an adverb, meaning “for a long time”:
  • Olen pitkään hereillä. = I’m awake for a long time.
  • He keskustelivat pitkään. = They talked for a long time.

Finnish often makes adverbs not with -sti but by putting the adjective in a case form. Here pitkään is a “case adverb” and is the normal, idiomatic form.
pitkästi exists but is rare and usually not used in this meaning; pitkään is what you want.

Both can mean “for a long time.”

• pitkään hereillä
• kauan hereillä

In many situations they are interchangeable:

• Olen pitkään hereillä.
• Olen kauan hereillä.

Nuances (very subtle, and often ignored in everyday speech):

• kauan is the most neutral, common way to say for a long time in general.
• pitkään can feel a bit more like for quite a long stretch, often relative to what you might have expected.

But for a learner, you can treat them as synonyms here. Native speakers freely use both.

Hereillä is a fixed expression meaning “awake” (not sleeping).

It is historically the adessive plural of an old word hereä, but in modern Finnish:

• hereillä is used almost only in the expression:
  • olla hereillä = to be awake

So:

• Olen hereillä. = I’m awake.
• Olemme yölläkin hereillä. = We’re awake even at night.

You cannot normally say:

• Olen hereä. (unnatural)
• Olen herä. (wrong)

Think of hereillä as a single lexical item (like one word) that you simply memorize with olla:

olla hereillä = to be awake

There is a near-synonym olla valveilla (also adessive plural), slightly more formal/literary.

Finnish has two main ways to talk about not sleeping:

1. olla hereillä = to be awake
   – describes the state of being awake

2. valvoa = to stay up / to be awake (often intentionally, or through the night)
   – describes actively staying awake, often with some effort

Your sentence:

• Lauantai-iltana olen pitkään hereillä.
  = On Saturday evening I am awake for a long time.

This focuses on the state (I’m awake, not asleep), and its duration.
If you wanted a stronger idea of deliberately staying up, you could say:

• Lauantai-iltana valvon pitkään.
  = On Saturday evening I stay up for a long time.
• Lauantai-iltana valvon myöhään.
  = On Saturday evening I stay up late.

Finnish usually uses the present tense where English uses will for the future.

So:

• Lauantai-iltana olen pitkään hereillä.
  can mean:
  • On Saturday evening I am (generally) awake for a long time.
  • This coming Saturday evening I’ll be awake for a long time.

Context (or additional words like ensi, “next”) clarifies whether it’s about a habitual fact or a future plan:

• Ensi lauantai-iltana olen pitkään hereillä.
  = Next Saturday evening I’ll be awake for a long time.

There is a future-like form (tulen olemaan), but it is used much less often than English will be, and sounds heavier or more formal/planned. The plain present covers most future uses.

Yes, you can—and usually should—leave minä out here.

Finnish is a pro-drop language: the personal ending on the verb already tells you who the subject is:

• olen = I am
• olet = you are
• on = he/she/it is

So:

• Lauantai-iltana olen pitkään hereillä.
  is completely natural and normal.

You add minä only when you want emphasis or contrast:

• Lauantai-iltana minä olen pitkään hereillä, mutta sinä menet aikaisin nukkumaan.
  = On Saturday evening I stay awake for a long time, but you go to bed early.

Yes. Finnish word order is quite flexible, and all of these are grammatically fine:

1. Lauantai-iltana olen pitkään hereillä.
2. Olen pitkään hereillä lauantai-iltana.
3. Pitkään olen hereillä lauantai-iltana. (more marked/emphatic)

The neutral or very typical pattern in everyday speech is to put the time expression first:

• Lauantai-iltana olen pitkään hereillä.

This answers the implicit question:

• When am I awake for a long time? → On Saturday evening.

If you move lauantai-iltana to the end, the sentence is still correct; it just slightly shifts the focus:

• Olen pitkään hereillä lauantai-iltana.
  might feel a bit more like clarifying which evening you mean.

Yes, and it changes the meaning slightly.

• lauantai-iltana = on (a/the) Saturday evening (one concrete evening)
• lauantai-iltaisin = on Saturday evenings (repeated, in general)

-isin here is a special ending used to express habitual / repeated time:

• aamuisin = in the mornings (usually)
• öisin = at nights / during the nights (habitually)
• lauantai-iltaisin = on Saturday evenings (as a habit)

So:

• Lauantai-iltana olen pitkään hereillä.
  = On Saturday evening I’ll be / I am (that one time) awake for a long time.
• Lauantai-iltaisin olen pitkään hereillä.
  = On Saturday evenings (in general) I’m awake for a long time.

Both are possible, but they mean different things:

• lauantai-iltana = on (a/the) Saturday evening
  – singular essive: one specific evening
• lauantai-iltoina = on Saturday evenings or on Saturday nights
  – plural essive: multiple evenings

So:

• Lauantai-iltana olen pitkään hereillä.
  → one particular Saturday evening (or one at a time)
• Lauantai-iltoina olen pitkään hereillä.
  → on (different) Saturday evenings, e.g. many of them over time

Compare:

• Kesäiltana on valoisaa. = On a summer evening, it is light. (generic but singular image)
• Kesäiltoina on valoisaa. = On summer evenings, it is light. (plural, repeated)