Iceland Poppies don't really come from Iceland. They come from Siberia, another place with extreme temperatures, but they'll bloom well in both hot and very cold climates if sown at the right times.
Plant poppies in early autumn and they'll flower at the beginning of winter. If kept well fed and watered, and the seed heads are nipped off, they will continue blooming until Christmas. They flower longer in a cool climate whereas hot weather and longer days means they put most of their energy into forming seed heads, not new flowers.
Sow seeds on top of garden beds and sprinkle them lightly with soil. They shouldn't be planted deeply. Make sure your chosen area is weed-free. If you're sowing from punnets of seedlings, plant them about a hand-span apart. You'll need about 20 plants per square metre for a thick display, as the stems are thin (and hairy). They also grow beautifully in big pots.
To get the most stunning and long-lasting display of Icelandic Poppies, pick off the first flower buds so that the plants are strong and sturdy before they bear their first blooms. Just twist off the tiny bulges in the middle of the rosette with your fingers.
The second trick is to water often, and feed them every week with a soluble fertiliser designed for maximum flower production. Once seedlings are about 10cm high, stop feeding them high-nitrogen, water-soluble fertilisers and swap to using high-potassium, water-soluble fertilisers. This will promote long flowering and slow down growth. If you garden organically, a mulch of good homemade compost will probably be all they need.