Snow drops can be planted at different times of the year.
In the autumn the dry bulbs can be planted and this is the cheapest method but not always the easiest; snow drops bulbs seem to be harder to get going then other bulbs. Few suppliers will be unable to give an assurance that the bulbs are freshly lifted. Bulbs which have been in garden centers for a long time will dry out, which does make them more difficult to grow from which is why buying "in the green" can be more successful, although more expensive
.Snowdrops are sold in pots during the winter and then can be taken out and potted up into the garden and this is a good way of getting them going. Growing Snowdrops in containers is not ideal because as with all container grown plants, they will be more prone to drying out which is fatal for snowdrops.
Probably the best time to plant snowdrops is in the spring, when they are planted "in the green" which is usually after flowering but whilst still in leaf which is around March time. This is not as cheap as bulbs, but less expensive than buying in pot and is a good way of getting snowdrops established.
If planting either established plants or in the green, plant to the same depth. This means planting so that the white part of the stem is underground. Given that they flower during January -February, the best time to buy plants (available at this time on line and mail order) "in the green" is March and April and plant out straight away.
Dormant bulbs can be planted in the summer, bulbs are always cheaper than plants but less reliable. The risk with bulbs, and for snowdrops, is drying out and so a moist soil and a mulch of leaf mould is a good growing medium. In addition, squirrels like the snowdrop bulbs as well.