Keep in mind when planting that Snappy mix is thought of as very hardy, so this plant will tend to survive through freezing conditions. Ensure your soil is loamy and has a ph of between 6.0 and 7.0 as Snapdragon is a weakly acidic soil to neutral soil loving plant.
By our calculations, you should look at sowing Snappy mix about 56 days before your last frost date.
Ensure that temperatures are mild and all chance of frost has passed before planting out, as Snappy mix is a very hardy plant.