Learn Russian: Из-за праздника в центре суета, нам совсем негде сесть. - Because of the holiday there is bustle in the center, we have absolutely nowhere to sit down.

This is the required structure for predicative negative adverbs like негде (nowhere), некуда (nowhere to go), and некогда (no time). These sentences are impersonal, so the person experiencing the lack is put in the Dative case (нам), followed by the adverb and an infinitive verb (сесть).

Why is the word for "nowhere" негде here instead of нигде?

Негде specifically means "there is no place (available)" to do something. It is always used with an infinitive like сесть, and it acts as its own negative, so you don't add the word не. Нигде simply means "not anywhere" and requires a regular conjugated verb with a double negative, for example: Мы нигде не сидим (We aren't sitting anywhere).

Why use the verb сесть instead of сидеть?

Сесть is perfective and means the specific action of "to sit down" or "to take a seat." Сидеть is imperfective and describes the ongoing state of "to be sitting." Since they are trying to complete the one-time action of taking a seat, сесть is the correct choice.

Why does праздник change to праздника at the beginning of the sentence?