Do you mean bought in plants or saved runners!

I find September/October a convenient time as the saved runners should be well rooted ready for transplanting!

Bought in plants can vary; sometimes the supplier will sell them in September/October or in spring as things begin to warm up!

I would recommend purchasing them in September / October as this gives them time to establish themselves prior to the onset of winter!

Spring purchases might not be convenient as the delivery date might be at a time when conditions are not suited to planting out, meaning the plants have to be kept in less than ideal conditions.

There are pictures here on how to secure runners in the event that you do not know how!

http://www.thegardenersalmanac.co.uk/Data/Strawberry/Strawberry.htm