Well, this is a first for me!! In all my years of growing and forcing bulbs, I have never heard of forcing bulbs to be in bloom after their normal bloom period. Typically, this is a process that is used to force bulbs to bloom early, not late. Was it some sort of kit? Did they come with any instructions?

To begin with, I think you may have difficulty in relying on any blooms at a specific date. You may have difficulty in relying on any blooms, period. The prechilled bulbs still need time to develop a root system and grow before any effort to bloom - very similar to other posts you see now regarding planting spring blooming bulbs that were overlooked in fall, even if maintained at proper cool temperatures (i.e., the prechilling).

My suggestion would be to pot them up right away - this is their normal growth season and unless you keep them at very low temperatures, they will attempt to grow anyway, soil or no soil. Keep them as cool as possible to hopefully slow development (just above freezing - a refrigerator may be necessary). Monitor their progress and when the foliage appears above the soil, move them out. Give them light and more warmth at this time. But delaying their bloom cycle for another 3 months is going to be very tricky. You may have better luck with the tulips than the hyaciths as many tulips can bloom quite late in the season, but hyacinths are much earlier bulbs.

You may want to contact the bulb suppplier for more specific instructions but I'm just very unsure that these bulbs are going to perform for you according to the schedule you had anticipated.

Organic, did the seller of the bulbs understand that you wanted to "force" these bulbs for flowering at a particular date?
Ordninarily bulbs are bought in the fall...and the seller is advised the nature of 'forcing' is wanted. They then sell you the type of bulb that is more in line with how you are bringing them to bloom. In other words, for forcing, they would sell you a bulb that forces well.

Normal bulbs require 14 to 15 weeks of cooling. Your pre-chilled then must only be required to 'fill in' the remaining weeks. The refrigerator then is the only place at this time you can hope for continuing the cooling process.
Just subtract the number of weeks in the fridge from the rquired 14. If your bulbs were given a pre-cooling of ....say 8 weeks, then you would be required to finish the remaining 6 in the fridge.
They need a temperature of between 35�F and 50�F...so the fridge is the ideal place. Be sure though to not put them into a crisper with other fruit.
Plus, these are living plants...bulbs. They must be given a medium to grow their roots in and they must be kept damp.
When they do acquire roots, then they are brought out to the sunshine which will target the production of bloom.

Into each pot, 3 hyacinths can be planted; 6 tulips in the other.
Put the bulb with the flat side toward the outer edge of the pot...this will produce the initial leaves pointing out..the first leaves will grow outward then from its neighbors.
If you are using clay potting, be sure to soak it overnight before placing soil into it. Fill each pot loosely with potting soil; the tops of the bulbs should be even with the rim. Don't compress them, just open the soil, place the bulb in. Then water. As the soil settles you can then add more soil but leave room for future watering. The pot must drain.
Label each pot so that you know who's who and when you planted it.

It does appear if you can give it the cooling required, your end of May date might just be possible. Delaying of bloom is brought about by denying sunlight, withdrawing so much water and otherwise put the bulb back in time to come to bloom.

I hope the guests appreciate how you have brought so much flowering into the day. If it works, you can then boast how you made it happen.

Good luck....but please, if you do make this happen, don't do what other parents have done...named their children some dopey name like May, or Flower, or Tulip....or Hope or Charity........

The time from sprouting to flowering varies by variety of tulip. The only reliable way to get blooms on a certain date it to get way more bulbs than you need and plant them sequentially (every 2 or 3 days) for a couple of weeks. That guarantees some will be blooming when you want. Additionally I've read that you can slow blooming by a few days by covering the flower but with a brown paper bag so it doesn't get any sun.

What worked for me with forcing tulips was: plant them with the tips sticking out of the dirt, under lights. If the bulb has a peel, remove it from the tip so the light can hit it. This will get the tulip bulb going the fastest. Be careful not to let the soil get too wet, potted bulbs are prone to rotting and flies. A layer of perlite or gravel on the top may help discourage flies.