A literary allusion is a reference within the text to something noteable outside of the text, often from other literature, but also from science, history, or even pop culture.
The book Night is full of allusions, most of which are religious references. At the very beginning of Section 5 Wiesel makes reference to Rosh Hashana (the Jewish New Year) and Yom Kippur (the Day of Atonement). There are several religious allusions within this section including reference to God as "the Almighty," "Thou," and "Master of the Universe." There is also reference to prayer and "bless[ing] his name," as well as the "Kaddish" which is the Jewish prayer for the dead. In addition to these religious allusions, several Biblical allusions are made, including the mention of Adam and Eve, Noah, the Flood, Sodom, and the "Last Judgement."