Breakdown of Hur ska ni fira den tolfte juli?
ni
you (plural, subject)
hur
how
ska
will
den
the (with en-words)
Questions & Answers about Hur ska ni fira den tolfte juli?
Why do we need the word den before the date, and where is the word for "of"?
In Swedish, dates are formed with the standalone definite article den (the) right before the ordinal number. You literally say "the twelfth July". You don't use any word for "of" between the day and the month like we do in English.
The number twelve is tolv. Why is the ordinal spelled tolfte here?
When turning tolv (twelve) into an ordinal number, the v changes to an f before adding the -te ending. This makes the word easier to pronounce, giving us tolfte (twelfth) rather than "tolvte".
Could ni be used to formally address just one person in this sentence?
No, ni is almost always the plural "you" (you all/you guys). While it used to be a formal singular "you" a long time ago, modern Swedish uses du for everyone. If you were asking just one person about their plans, you would say: Hur ska du fira den tolfte juli?
Why does the sentence use ska instead of kommer att for the future tense?
Both can express the future, but ska implies an intention, a plan, or a decision. Since you are asking people how they plan to celebrate, ska is the most natural choice. Kommer att is usually for predictions or things outside of a person's direct control.