What are examples of allusions in Frederick Douglass' speech, "What to the Slave is the Fourth of July?"

An allusion is a literary device which references events, characters, or ideas of political, historical, or religious significance.

In Frederick Douglass' speech, an example of an allusion can be found in his reference to Passover. Each year, Jews celebrate Passover as a commemoration of Jewish emancipation from Egyptian rule. The reference is a biblical allusion that serves to emphasize Douglass' understanding of the importance of July 4th to his audience:

This, to you, is what the Passover was to the emancipated people of God. It carries your minds back to the day, and to the act of your great deliverance; and to the signs, and to the wonders, associated with that act, and that day.

Frederick Douglass further equates the signs and wonders associated with the act of deliverance from British rule with the signs and wonders which preceded the Jewish exodus from Egypt. Using allusions is a clever way for Frederick Douglass to endear himself to his listeners and to easily provide his audience with a potent context to understand his appeals to their sense of justice. Later, he also uses another biblical allusion to draw attention to the characteristic intransigence of the British:

But, with that blindness which seems to be the unvarying characteristic of tyrants, since Pharaoh and his hosts were drowned in the Red Sea, the British Government persisted in the exactions complained of.

Above, Frederick Douglass characterizes the British as tyrants, not far removed from the Pharoah of Egypt who managed to enslave a whole race of people. The allusion is calculated to inspire strong emotions in his listeners and to instill in them an understanding of his people's desire to be free.

Another allusion in the speech is also a biblical one:

By the rivers of Babylon, there we sat down. Yea! we wept when we remembered Zion. We hanged our harps upon the willows in the midst thereof. For there, they that carried us away captive, required of us a song; and they who wasted us required of us mirth, saying, Sing us one of the songs of Zion. How can we sing the Lord’s song in a strange land? If I forget thee, O Jerusalem, let my right hand forget her cunning. If I do not remember thee, let my tongue cleave to the roof of my mouth.

This is a reference to Psalm 137, what many people would call an imprecatory psalm. It's the kind of psalm that illustrates a call to God to rain judgment down on those who oppress the defenseless and the innocent. Here, the psalm is a lament by the Jewish people who have been exiled to Babylon. They are once more slaves to a foreign power and are helpless, destitute, and homesick. In their suffering, the Jews proclaim that they can never 'sing the Lord's song in a strange land.'

In the same way, Frederick Douglass explains to his audience that his fellow African-Americans can never fully celebrate July 4th while still living in slavery. They must be free to fully appreciate the significance of Independence Day.

Fellow-citizens; above your national, tumultuous joy, I hear the mournful wail of millions! whose chains, heavy and grievous yesterday, are, today, rendered more intolerable by the jubilee shouts that reach them. If I do forget, if I do not faithfully remember those bleeding children of sorrow this day, “may my right hand forget her cunning, and may my tongue cleave to the roof of my mouth!” To forget them, to pass lightly over their wrongs, and to chime in with the popular theme, would be treason most scandalous and shocking, and would make me a reproach before God and the world. My subject, then fellow-citizens, is AMERICAN SLAVERY.