Dahlias are tender tubers.
If you plant them out before the frosts are over, they may get frosted and die, so pot them up in March or early April, in a generous (at least 2 – 3 litre pot) filled with multi-purpose potting compost.
Place them in a light, frost-free place and keep the compost moist. They will have formed bushy plants by the time the frosts have ended and will be in flower by the beginning of July.
If you want more Dahlia tubers there is a really easy way of getting more free plants and this video shows you how to do it.