Call it folklore, but most Maine gardeners wouldn't think about planting anything until after the full moon in May, as long as it's after the 20th of the month. Dahlias have a tender bulb, which means they can't be left in the ground over the winter, so plan on putting them into the ground after the 20th of May or the full moon, whichever occurs last.
Your soil should clump together slightly; if it is more like mud than chocolate cake, turn the soil over with a shovel, and wait another week. As soon as the soil feels like chocolate cake, it is time to plant the dahlia bulbs.
Plant the bulb (or tuber, as they are sometimes called) with the "eye" facing up. The eye is opposite the end with fine roots. Plant the bulbs 6 to 7 inches down, and cover them with soil. Don't forget they need to be pulled out of the ground before the first frost in the fall.