Learn Hardcore Finnish: Vappu on iloinen juhlapäivä, jolloin kadut täyttyvät musiikista ja vappupalloista. - May Day is a joyful holiday when the streets fill with music and May Day balloons.

In Finnish, a comma is normally placed before a relative clause, and jolloin kadut täyttyvät musiikista ja vappupalloista is a relative clause describing juhlapäivä.

The structure is:

Vappu on iloinen juhlapäivä,
jolloin kadut täyttyvät musiikista ja vappupalloista.

So the comma works much like in English:
“Vappu is a happy holiday, when the streets fill with music and May Day balloons.”
That “when the streets fill…” part is separated by a comma because it’s extra information about the holiday.

What does jolloin mean, and how is it different from kun?

Jolloin is a relative adverb meaning roughly “when (at which time)”. It refers back to a time expression – here, to juhlapäivä (holiday).

Vappu on iloinen juhlapäivä, jolloin kadut täyttyvät…
= “Vappu is a happy holiday, when (on which day) the streets fill…”

Kun is a conjunction meaning “when / as / if” and does not itself refer back to a specific noun:

Kun on Vappu, kadut täyttyvät…
= “When it’s Vappu, the streets fill…”

So:

jolloin = “at which time”, pointing back to juhlapäivä
kun = “when”, introducing a time clause without that relative reference.

Why is juhlapäivä in the basic form (nominative), not juhlapäivänä?

After the verb olla (“to be”), Finnish can use either:

nominative for simple classification: “X is (a) Y.”
essive (-na/-nä) for “as / in the role of / in the state of”.

Vappu on iloinen juhlapäivä.
Here juhlapäivä is in the nominative: “Vappu is a happy holiday.” (It tells what Vappu is.)

If you said:

Vappu on iloisena juhlapäivänä…
that would sound more like “On Vappu, as a happy holiday, …” describing Vappu as a state or role during that time. That’s a different nuance.

So nominative is correct here for a straightforward “X is Y” statement.

What’s the difference between juhla and juhlapäivä here?

Juhla = “celebration, party, feast, festival” (more about the event/celebration itself).
Juhlapäivä = “holiday, festive day” (literally “celebration‑day”; focuses on the day as a holiday).

Both can be used for many occasions, but:

Vappu on iloinen juhla. = “Vappu is a happy celebration/festival.”
Vappu on iloinen juhlapäivä. = “Vappu is a happy holiday (day off with celebrations).”

In this sentence, juhlapäivä nicely emphasizes that Vappu is an actual holiday date.

Why is kadut in the plural?

Kadut is the plural nominative form of katu (“street”).

Finnish often uses the plural for things that naturally occur as a set:

kadut – the streets (of a town)
ihmiset – people
puut – the trees

Here, kadut is the subject of the verb täyttyvät (3rd person plural), so they agree:

kadut täyttyvät = “the streets fill (up)”

You could say katu täyttyy (“the street fills”), but that would mean just one street, which is not what we want here.

What is the verb täyttyvät, and how is it different from täyttävät?

The base verbs are:

täyttyä = “to be filled, to become full” (intransitive)
täyttää = “to fill (something)” (transitive)

In the sentence:

kadut täyttyvät musiikista ja vappupalloista
→ täyttyvät is the 3rd person plural of täyttyä (“are filled / become full”).
Literally: “the streets become filled with music and May Day balloons.”

By contrast:

He täyttävät kadut musiikilla ja vappupalloilla.
→ täyttävät (from täyttää) means “They fill the streets with music and balloons.”

So:

täyttyvät = the streets fill themselves / become full (no explicit agent)
täyttävät = someone fills the streets.

Why are musiikista and vappupalloista in the -sta/-stä form?

Musiikista and vappupalloista are in the elative case (-sta/-stä), which often means “out of / from”, but with täyttyä it has a special function:

The pattern is:

täyttyä + elative = “to be filled with (something)”

So:

kadut täyttyvät musiikista = “the streets are filled with music”
kadut täyttyvät vappupalloista = “the streets are filled with May Day balloons”

The idea is “they become full from music and balloons”, which in natural English is expressed as “full of / with music and balloons.”

Could you use the basic form, like musiikki and vappupallot, instead of musiikista and vappupalloista?

With täyttyä, that would sound wrong or at least unidiomatic. The verb täyttyä expects its content with the elative case.

Compare:

Kadut täyttyvät musiikista ja vappupalloista. ✅
Kadut täyttyvät musiikki ja vappupallot. ❌ (ungrammatical)

If you want musiikki and vappupallot in basic form, you would change the verb and structure:

Kaduilla on paljon musiikkia ja vappupalloja. – “There is a lot of music and balloons on the streets.”

But for “the streets fill with X”, you keep täyttyä + elative.

Why is the verb täyttyvät in the present tense if this happens only on Vappu?

Finnish often uses the present tense to describe repeated, regular or typical events – similar to the English “present simple”:

Vappu on iloinen juhlapäivä, jolloin kadut täyttyvät…
→ “Vappu is a happy holiday when the streets (typically) fill with music and balloons.”

This doesn’t mean it is happening right now; it describes what usually happens on that day. English does the same:

“On Sundays, the shops are closed.”
“Every Christmas, we visit my grandparents.”

So the Finnish present describes a general habit / typical occurrence.

How does adjective agreement work in iloinen juhlapäivä?

In Finnish, adjectives agree with the noun in:

number (singular/plural)
case

Here:

juhlapäivä is singular nominative
iloinen is also singular nominative

So they match: iloinen juhlapäivä (“a happy holiday”).

If you changed the case or number, both would change:

iloisena juhlapäivänä – “as a happy holiday” (both in essive)
iloiset juhlapäivät – “happy holidays” (both plural nominative)

That matching is what we call adjective agreement.

Could you say Vappu on iloinen juhla, jolloin kadut täyttyvät… instead? Would it change the meaning?

Yes, you can say:

Vappu on iloinen juhla, jolloin kadut täyttyvät musiikista ja vappupalloista.

This is perfectly grammatical. The nuance is slightly different:

juhlapäivä emphasizes it as an official / calendar holiday
juhla emphasizes the festive celebration itself

In practice, both would usually be understood in almost the same way in this sentence.