In What S+V,

To her frustration, she had been at a loss for a way of contacting me. Bright and early on the day when her own wedding ceremony was about to happen in a few hours, she came across an ad promoting my new book and wasted no time in e-mailing me. Doubting that just a few hours away from her own wedding ceremony, a bride could have the presence of mind to e-mail me, I repeatedly read her e-mail to get her wedding date right. In what laid my doubt to rest, she read something into the timing of her knowledge about my contact information when she said that on one of her holiest dates in her life she got to know of it, something for which she had so much longed.

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Could I substitute in what laid my doubt to rest with laying my doubt to rest?

P.S. Could you tell me that (as in she said that on one of her holiest days...) refers to what?