Min systers födelsedag är den sjätte mars.

Because min systers means my sister’s.

In Swedish, possession is often made by adding -s to the owner:

• min syster = my sister
• min systers = my sister’s

So:

• Min systers födelsedag = My sister’s birthday

This works a lot like English 's, but in Swedish you usually just add s with no apostrophe.

Swedish does not normally use an apostrophe for possession.

So English:

• my sister’s birthday

becomes Swedish:

• min systers födelsedag

not min syster’s födelsedag

The possessive ending is simply -s.

Swedish often makes compound nouns, where English would use two words.

So:

• födelse = birth
• dag = day
• födelsedag = birthday

This is very common in Swedish. English learners often want to separate words, but Swedish usually joins them into one noun.

Other examples:

• sjukhus = hospital
• sommarlov = summer holiday / summer break
• köksbord = kitchen table

Because Swedish usually does not add an article when a noun is already part of a possessive phrase.

Compare:

• födelsedag = birthday
• min systers födelsedag = my sister’s birthday

English also does this:

• not my sister’s a birthday
• not my sister’s the birthday

So once the possessor is there, no article is needed.

In Swedish, when you say what date something is, you normally use är directly with the date expression.

So:

• Min systers födelsedag är den sjätte mars.

Literally, this is like:

• My sister’s birthday is the sixth of March.

English often prefers on in some situations, but Swedish does not need på here.

You would use på in other kinds of time expressions, but not usually when simply stating the date after är.

That is the normal Swedish way to say a calendar date.

Pattern:

• den + ordinal number + month

Examples:

• den första januari = the first of January
• den tredje maj = the third of May
• den sjätte mars = the sixth of March

So sjätte is an ordinal number meaning sixth.

Because dates use ordinal numbers, not cardinal numbers.

• sex = six
• sjätte = sixth

In English, you also usually say:

• March sixth or the sixth of March

not:

• March six

So Swedish does the same kind of thing.

In standard Swedish, dates are usually said with den before the ordinal:

• den sjätte mars
• den tionde april
• den tjugofjärde december

So den is a normal part of the date expression.

You may sometimes see dates written in numbers, like 6 mars, but when fully said with an ordinal, den is standard.

In date expressions, Swedish normally uses den with the ordinal:

• den sjätte mars
• den första juni

This is just the conventional form for dates. It is not really chosen because mars has one gender or another in the same way you might expect from ordinary noun agreement. It is simply the fixed pattern used for dates.

So it is best to learn:

• den + ordinal + month

as one complete structure.

Normally, no.

In Swedish, months are usually written with a lowercase letter:

• mars
• april
• december

So den sjätte mars is correct, not den sjätte Mars.

This is different from English, where months are capitalized.

Yes. In writing, Swedish often uses numerals:

• 6 mars

But when speaking fully, people commonly say:

• den sjätte mars

So both are useful:

• written: 6 mars
• spoken/full form: den sjätte mars

The sentence follows a very normal Swedish pattern:

• Min systers födelsedag = subject
• är = verb
• den sjätte mars = complement

So:

• Min systers födelsedag är den sjätte mars.

This is straightforward subject + verb + information about the date.

It works much like English:

• My sister’s birthday is the sixth of March.

The tricky part is the beginning sj-, which has a sound that does not exist in standard English.

A rough learner-friendly approximation is something like:

• SHET-teh

But that is only approximate. The real Swedish sj sound is more distinct and varies somewhat by dialect.

A few tips:

• ä sounds a bit like the e in bet, but more open
• the tt makes the consonant short and clear
• the final e is lightly pronounced

So sjätte is approximately:

• SHET-teh
  with the stress on the first syllable.

Yes. Grammatically, it is the same idea.

• Min systers födelsedag = my sister’s birthday
• more literally: the birthday of my sister

But in natural English, my sister’s birthday is the normal translation.

In Swedish, the -s possessive construction is very common and natural here.