As Susan Henchard lies on her deathbed, she figures it's time for confession. Not long before she passes from this world into the next, she confesses that she was the one who sent the matching notes to her daughter Elizabeth-Jane and the young Scotsman Donald Farfrae. She makes this confession to her daughter in person.
In doing so, she had no intention of making fools out of them, but rather of bringing them together. By sending them matching notes, she hoped that they would both turn up at the granary at the same time, thereby giving them the opportunity to get to know each other, thus potentially setting them on the road to marriage.
But there's an even bigger confession in store, one that Susan makes to her husband Michael in a letter. In that letter, she candidly confesses that Elizabeth-Jane is not his daughter but Newsom's. Henchard's Elizabeth-Jane died in infancy. All this accounts for why Susan, upon remarrying Michael, didn't want Elizabeth-Jane to change her surname from Newsom to Henchard, as Newsom was her biological father.
The contents of the letter are so incendiary, so potentially scandalous, that the letter is not to be opened until Elizabeth-Jane's wedding day. When Michael discovers the truth, he starts behaving coldly towards Elizabeth-Jane, now that he knows she isn't really his daughter.