Keep in mind when planting that Bells mix is thought of as very hardy, so this plant will tend to survive through freezing conditions. Ideally plant in loamy soil and try to keep the ph of your soil between the range of 6.0 and 7.0 as Bells mix likes to be in weakly acidic soil to neutral soil.
By our calculations, you should look at sowing Bells 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 Bells mix is a very hardy plant.