As soon as you get your plants open the box up and remove them from the packing. This stops them from 'sweating'. Depending on what time of year, and the weather conditions, this is what you should do:
It is best to plant them as soon as you can, especially if you are buying plants during the summer months. If you are buying perennials in spring they are most likely to have been in the pots since the summer which means many of the nutrients in the soil will have been used up. Therefore it is important to plant them as soon as you can, especially if they are just starting to grow.
In most cases our plants are sent out in 9cm pots and the pots can be let for quite a time before planting as long as they do not dry out.