It is an example of “reverse symbolism” because Fourth of July is Independence Day, and Rufus called Dana back to a time when she would be considered a slave.

Symbolism is when something stands for something else.  In this case, Independence Day does not stand for freedom, but instead for slavery.  This is why Dana calls it reverse symbolism.

Dana has gotten used to being called back to the past every once in a while.  Yet she has no control over when she goes, how long she stays, or when she gets to leave.  She knows that she can only be called when Rufus’s life is in   danger, so whenever she comes to the past she will be in a difficult situation.

We had fifteen full days together this time.  I marked them off on the calendar...  With some kind of reverse symbolism, Rufus called me back on July 4. (“The Rope”, part 2, p. 243)

When Dana goes back in time, she has no freedom at all.  She has to pretend she is a slave, and try to find ways to cope with living in a time when she has no rights. 

Ironically, since this is the last time Rufus will call her, she actually will become free as a result.