Earlier in the novel, the narrator, Hazel, tells of her "Cancer Miracle": she was about to die, but the (fictitious) drug _Phalanxifor_ made her tumors shrink and "resulted in a bit of purchased time".
So when Hazel refers to the time "pre-Miracle" in your quote, she means before that successful treatment, meaning (I guess) that her parents took her to Disneyland when they thought she was going to die.
_I found the answer to your question by searching for "miracle" in the_ Look inside _feature on Amazon. **de** (the German site, because the relevant page did not display to me on Amazon.com)._