Is the first week the worst week?
Worst? In what way? There is the physical withdrawal aspect, which is worse when you first quit. But from a recovery standpoint, it's the easiest to stay quit during, as the memory of our last escapade is usually fresh in our mind and our commitment to sobriety is strong.
From that perspective, the hardest week might be when you start feeling good, start thinking you don't have a problem, or when life takes a downturn and you haven't figured out how to cope other than return to the bottle. So I'd say the worst week is the week you decide it's okay to drink.