Sidste år faldt påsken i marts, men næste gang falder den i april.

Because Danish main clauses follow the V2 rule: the finite verb usually comes in second position.

So when the sentence starts with Sidste år as the first element, the verb must come next:

• Sidste år faldt påsken i marts.

If you start with the subject instead, then you get:

• Påsken faldt i marts sidste år.

Both are correct, but the word order changes depending on what comes first.

For the same reason: Danish still uses V2 word order in main clauses.

The first element is næste gang, so the finite verb comes second:

• næste gang falder den i april

If you put the subject first, it would be:

• Den falder i april næste gang

That version is grammatically possible, but it sounds less natural in this context.

Påsken is the definite form of påske.

In Danish, holidays are often used in the definite form when talking about the event itself:

• julen = Christmas
• påsken = Easter

So påsken here means Easter as a specific holiday period. Using påske without -n is possible in some expressions, but in a sentence like this, påsken is the normal choice.

Because påske is a common-gender noun in Danish, and the pronoun for that gender is den.

So:

• påsken → den

This works just like referring back to a noun in English with it, except Danish distinguishes between common gender (den) and neuter (det).

Faldt is the past tense of falde, so it fits sidste år.

For the future part, Danish very often uses the present tense to talk about future events when the time is clear from context. So:

• næste gang falder den i april

means next time it will fall in April, even though Danish uses the present form falder.

This is very common in Danish.

Yes, but this is an idiomatic use. Danish uses falde in expressions about where a date lands in the calendar.

So:

• Påsken faldt i marts = Easter fell in March
• Julen falder på en mandag i år = Christmas falls on a Monday this year

It is very similar to English falls in sentences like Easter falls in April.

Because Danish normally uses i with months:

• i januar
• i marts
• i april

So i marts means in March, and i april means in April.

It means next time or the next occurrence.

In this sentence, it refers to the next time Easter lands in the relevant way being discussed. It is a very common expression:

• Denne gang = this time
• Næste gang = next time
• Sidste gang = last time

Because in Danish, names of months are normally written with lowercase letters.

So Danish writes:

• marts
• april
• mandag

not:

• March
• April
• Monday

This is different from English.

Yes, that would be understandable and grammatically fine.

But faldt is more idiomatic when talking about where a movable holiday lands in the calendar. It sounds more natural in this context:

• Påsken faldt i marts = more idiomatic
• Påsken var i marts = understandable, but less precise/stylish

So the original sentence uses the more natural Danish phrasing.

Because men means but and joins two main clauses:

• Sidste år faldt påsken i marts
• men næste gang falder den i april

In standard Danish writing, a comma is used before men when it connects clauses like this.