Why do the Matariki stars only show at a certain time of year?

LEVEL: Intermediate / Secondary.

A good way to start researching this question is to check for earlier Many Answers entries on the topic of Matariki. Our entry on Matariki - Māori New Year doesn’t answer the specific question, but it has great background information and with a bit of hunting you can find some useful clues:
• Description of the appearance of Matariki in the dawn sky in late May or early June.
• Most common name for the Matariki stars is Pleiades.  

The best information about this is in the diagram and explanation in the booklet produced by the Māori Language Commission in 2010. You can download a copy of it here. In the booklet you will find the technical term used in astronomy - “heliacal rise” of stars. 

Now you can use heliacal rise or heliacal rising as search words or keywords to search on Google or on the web. 

HOT TIP: A good place to look is Wikipedia which has great coverage of many topics, including heliacal rising of stars.  But you need to be aware that this information is contributed by lots of different people.  If you are using this site, it always pays to check the information against that on other sites or in books from the library.  Also remember to check the references at the bottom of the Wikipedia article.