Explain "...seven times cursed and seven times sealed."

I'm not an authority on Judaism but I think this has to do with the Jewish cycle of life and creation. There are seven days of creation, the seven days of mourning after death, the seven blessings said at a wedding. It sounds like he is saying his time in the camp will be the completion of all the cycles. 

Wiesel uses imagery and allusions that are shared between the Bible and Talmud. Aslan's answer is generally correct. The first use of this phrasing can be found in Genesis 4:15:

God declared to Cain, "Now you are under a curse and driven from the ground, which opened its mouth to receive your brother's blood from your hand. When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth" (Genesis 4:11-12). In response, Cain lamented, "My punishment is more than I can bear. Today you are driving me from the land, and I will be hidden from your presence; I will be a restless wanderer on the earth, and whoever finds me will kill me" (Genesis 4:13-14). God responded, "Not so; if anyone kills Cain, he will suffer vengeance seven times over." Then the Lord put a mark on Cain so that no one who found him would kill him" (Genesis 4:15-16).

Instead of the "Mark of Cain' the jews of Sighet had to wear the yellow star, but unlike Cain's mark, the yellow star seals the fate of the jews instead of offering them protection. Elie's father was either hopefull or mistaken when he said "The yellow star? Oh well, what of it? You don"t die of it..." However, as a jew in Auschwitz states later, "Hitler keeps his promises."