Actually both methods--starting to count the day after you set versus they hatch on the same day of the week as you set them are both 21 days and just 2 different ways of counting the same thing.
Take April as an example:
Method 1--If you set them Friday, April 1st, you would skip ahead 3 weeks and look at that Friday, which is April 22nd as your hatch day.
Method 2--If you set them Friday, April 1st, you would start counting 21 days on Saturday April 2nd. That would give you Friday April 22nd as your hatch day (1+21=22, or you can manually count them on your calendar).
Same difference. I think the week counter looks quicker (I don't have to mess up any simple math--which I am very good at doing). It's also easier if you are skipping between months.