I think that Wiesel understands the significance on a couple of levels in the timing of the deportations and Nazi intrusion into Sighet.  On one hand, it reflects the sadism of the Nazis in disrupting one of the holiest of occasions in the Judaic faith and bringing the followers of the religion to death at that specific moment.  Wiesel does not hesitate in displaying the sadism of the Nazis in so many moments in the narrative that to bring this out in the opening is almost a foreshadowing of what is to come.  From a thematic point of view, Wiesel is also able to evoke the questioning of spiritual faith that will dominate the work.  The structure of the narrative is one where the questioning of faith is part of the work's structure and part of the narrative's essence.  There is a questioning of God and the paradigm of religious salvation throughout the narrative.  Part of the Nazi cruelty is to, in a sense, "murder" the notion of God for a community that placed so much stock in the power of the divine.  The arrests and deportations that happen during Passover is merely one part of this chain or sequence where faith and spiritual identity were obliterated in the shadow of the gas chamber.