What is the irony of the date on which Hermia must give her decision about her future, her love, and possible marriage?
Theseus and Hippolyta will be having an engagement party celebrating their love while Hermia will have to make a choice effectively alienating her from her love.
Hermia has chosen death instead of marriage to Demetrius, so her new path in life will actually be death.
On the very day celebrating her first meeting with her love, Hermia will have to choose a path that will separate them forever.
Theseus is impatient to decide this unpleasant business and get it behind him because he wants to concentrate on his own love and marriage. One would think that Theseus would be more sympathetic to Hermia and her plight than condemn her to a type of death the very next day.
On the very day that Theseus will be celebrating his union to his love, he is forcing Hermia to choose a path that will separate her from her love.