Learn Hardcore Finnish: Kesän perinne on juoda kahvia puistossa. - The summer tradition is to drink coffee in the park.

In Finnish, when one noun attributes another—here kesä (“summer”) describing perinne (“tradition”)—the attributive noun goes into the genitive. So Kesän perinne literally means “the tradition of summer.”

Why does the sentence use the infinitive juoda rather than a conjugated form like juomme?

After the copula on (“is”), Finnish commonly uses the first infinitive to express “to do something” as the predicate. Thus perinne on juoda translates to “the tradition is to drink…”

Why is kahvia in the partitive case?

The partitive (here marked by -a) indicates an indefinite or ongoing quantity. With verbs of consumption like juoda, you use the partitive when you mean “drink some coffee” in general, not consume a whole/defined amount.

What role does puistossa play and why the -ssa ending?

Puistossa is in the inessive case (-ssa), which marks location “in” something. Here it answers “Where do you drink?” – “in the park.”

Could we rephrase this with an että-clause instead of the infinitive?

Yes. A more explicit version is:
Kesän perinne on, että juodaan kahvia puistossa.
This uses että (“that”) plus the passive/indefinite juodaan (“one/people drink”). It’s grammatically correct but a bit heavier—Finns often prefer the concise infinitive form.

What if I want to talk about several summer traditions instead of just one?

You’d use the plural perinteet and a different structure, for example:
Kesän perinteisiin kuuluu kahvin juominen puistossa.
Here perinteisiin is the inessive plural of perinteet, kuulua means “belong to,” and kahvin juominen turns the action into a noun phrase.