Learn Swedish: Var inte spontan! Den andra sittningen börjar först på kvällen. - Don't be spontaneous! The second seating doesn't begin until the evening.

You are right that först usually means 'first'. However, when talking about time, Swedish uses först to indicate that an event happens later than expected, which translates to 'not until' in English. Notice that unlike English, Swedish does not use a negative word like 'not' in this structure.

Why does the sentence use Den andra sittningen with both the word Den and the -en ending on sittning?

This is a great example of the Swedish double definite rule. When a definite noun is modified by an adjective, like the ordinal number andra here, you must use the standalone front article Den as well as the definite suffix -en on the noun itself.

Does sittning literally mean 'sitting'?

Yes, sittning is a noun formed from the verb sitta. In a restaurant context like this one, it translates perfectly to 'seating', referring to a specific time slot for guests to eat. As a fun cultural note, a sittning is also the standard term for a formal dinner party at Swedish universities.

Why is the command Var inte spontan instead of Vara inte spontan?