The famous first sentence of A Tale of Two Cities is 119 words long. Dickens uses antithesis to make contrasting lists. In these lists, he foreshadows the major themes and events of the books.
First of all, at some times things go well, and at some times they go badly. There are happy moments in the book, and moments of great tragedy. Dr. Manette is freed from prison, but suffers from Post-Traumatic Stress. Lucie and Charles get married, but Darnay is in danger. Thus, it is simultaneously “the best of times” and “the worst of times” for the family.
…it was the season of Light, it was the season of Darkness, it was the spring of hope, it was the winter of despair, we had everything before us … (Ch 1, p. 4)
There is both wisdom and foolishness. Some characters make very wise choices, and others downright stupid ones. Why did Darnay wait to tell Dr. Manette who he really was? It turns out to have been wise. Should Carton have stalked them and enjoyed their love and happiness by proxy? It worked, because he was able to take Darnay’s place at the Guillotine. Yet terrible choices are also made. Dr. Manette should never have questioned the St. Evremonde brothers. He ended up missing most of his daughter’s life rotting in prison. The Manettes never should have returned to Paris.
Thus, there is both darkness and light for the main characters as they experience joy and despair, hope and pain, throughout the course of the book. Dicken's first sentence foreshadows this.
it was the best of times vs. it was the worst of times - Dr. Mannette and Lucy, Darney and Lucy, vs. The French Revolution