Not a silly question. Dahlias are a breed of their own.

Most of the answer you seek is because some dahlias 'sprout' in early spring and are called early bloomers. Meaning (according to your conditions) anywhere from April to june.

Most dahlias are summer dahlias and then there are the late bloomers. I have 4-5 late bloomers still blooming and it appears a couple will last into Nov.

Under your conditions, you could pretty much plant anytime you wish. The only bad times would be trying to plant an early bloomer in March.But you could plant summer and late bloomers that late with little downside.

As for catalog ordering, most cats we all peruse (Swan Island,Dan's,Amer. Mdws will say 'sold out' or out of stock until Summer.They won't accept orders usually until near fall and they ship sometimes as late as March to CA. So 'their' dahlias may well say plant in Spring since they don't intend to get them to you until then.

Hope this helps.