As Erel already answered correctly, this is not the expiration date. But make sure to check the expiration date. it should be somewhere on the box!
If I understand correctly, it means that religious Jewish people should avoid the task of opening the box on Saturday.
It means it will never taste as good if you open it in other times.
this is not 100% correct.
i can open the box or a snack on Saturday if the opening is destroying the package and also make sure not to cut it between the letters.
the reason for that is:
there are things that are not allowed to do it Shabbat. (Saturday)
one of them is DELETE (the opposite of write, i think it is called delete or clear?!)
and if you cut it between the letters it is like deleting the word
the second problem is that you may not BUILD something even by destroying it. For example, if you open a Tuna box in Shabbat there is no problem but if you want to use this box to store something after Shabbat it turns out that you created a box even though you opened the box to eat the Tuna in the end if you want to keep the box for another purpose it is not allowed. if you throw it in the garbage then it is ok.
the is no use of the Mazot box so there should not be a problem opening it in Shabbat although if you can open it before then do it so this is basically why it is written.