If you visit your garden centre regularly, then you’ll probably see many offers around for buying bulbs at this time of year, but that doesn’t mean this is the correct time for planting bulbs. You should take advantage of the offers, and then spend the next month preparing the ground for your flower bulbs to be planted at the beginning of Autumn.
There is a lot of variation among the different flower species, so it is worth doing some investigation into the specific time frame, but, generally speaking, planting it between the end of September and the end of November will ensure that the bulb has enough time to gather some energy in ideal conditions so that it can quickly convert that energy into producing a flower for Spring.
If you want to ensure that your flowers grow well, then you need to prepare the soil. This is just as important as ensuring you plant at the correct time.