From experience, this doesn't always happen. Yes, it DOES happen, but more often than not, its a myth that it will happen to you. If you shop only for the Hallelujah moment, hereafter referred to as the HM, you will be shopping past the wedding. On the reverse side, I've seen HM's worthy of a televangelist and the bride chose another dress based on whim.
For some brides, it comes down to a matter of personal preference. They look good in several styles, they have to pick the one that the like best. Other brides look great only in one style and they have to narrow their focus. For others, its a matter of budget.
Reasons aside, don't wait for the HM to make a decision on your wedding dress. If you love it and you feel the dress, then you should buy it. Go on to another decision, cross it off your list and shop for accessories. Once you find what you love, STOP SHOPPING and don't second guess your decision. You'll only make yourself and those around you crazy. Indecision is a handicap. A bad decision is better than no decision.
You ideally need to order 9 to 12 months ahead of your wedding date to have your dress made and arrive in time for you to start your alterations. All wedding dresses need some form of alterations, you can't just put it on and head to the church. Alterations take time. If you need to order a dress on rush, expect to pay extra. Don't expect because you CAN order a dress on rush that the manufacturer can accommodate your schedule. It doesn't always happen. Avoid disappointment and order timely.
The bridal manufacturers can't produce a gown in a matter of days. These dresses are all hand beaded, every tiny bead and crystal is sewn on by hand. It takes time for fine workmanship. From production time, add shipping time and time for it to clear US customs as most dresses are manufactured in the far east.
Bottom line, order in time for your dress to arrive WITHOUT stress and in time for alterations. If you have had a HM, congratulations. If you haven't, don't live for it. It may never happen, but in all probability, you'll be a beautiful bride in a dress you love!