Mary,

Actually you can plant them outside now though I am curious as to what zone your in that they would bloom in August?

What kind of Oriental lily bulbs did you buy? This little chart is for Zone 4 from the U of MN. You should probably have a back up plan. If you're just planting this spring, you may get good foliage growth but no flowers this season. You can plant now as long as the soil is workable and not waterlogged. I plant lilies in the fall, but the only ones I can count on to produce blooms all through August and well into September are the true tiger lilies.

"Black Beauty" Oriental dark red (July/August)
"Journey's End" Oriental deep pink (August)
"Stargazer" Oriental crimson-red (August)
"Yellow Ribbons" Oriental white/yellow (August)
"Casa Blanca" Oriental pure white (August/September)

I lived in Nebraska which is zone 4 and ours lilies didn't bloom that late. So that's interesting to know.

I thought the chart was a bit optimistic myself - seems here in northern MN (or at least out in the real world and out of the test gardens), the big lily bloom is in July. Except for the Lilium Tigrinim. I do think the OP is going to be disappointed and should definitely have a back up plan.

Instead of buying containers, is there any chance you know someone like me who has a BUNCH of nursery containers that I didn't throw away last year?

Many nurseries recycle old plastic containers and give them away for free. Or check your local Craig's list or freebay.

Planting lilies now should result in blooms this season - no reason not to :-) Many lily bulbs are sold in spring for planting now for summer blooms. And lilies are not frost sensitive - you do not have to wait for last frost date to plant out. You only need soil that is workable (not frozen or overly saturated by rain or snowmelt).

FWIW, I did a very similar container planting of lilies (Casa Blanca) for a wedding several years ago - the wedding was in late July and the lilies bloomed on schedule.

Late July I can see but August is pushing it.

The only lily that blooms in August in my garden is Lilium spaciosum rubrum. Others would be long gone.

Marys,
I agree that you should get them into the ground, they can take some frost and unless the ground is too wet or frozen that is the best place for them to be. You could leave some in pots as well to move around the yard when the wedding happens, provided they are blooming at that time.
Orientals for me usually bloom in August and usually bulbs planted in the spring bloom later the first year so you maybe okay for timing of bloom....lots of factors can affect this. It is not a bad idea to have some sort of back up plan in case, but sounds like it will be lovely if all works out.
Flora