You're assuming that the effect (i.e. the protection) activates the same way as it deactivates, but that is not necessarily the case. To prove that point, I'll use a real-world example: ketosis. Coincidentally, it uses the same week-long period as the wolfsbane.
A keto diet consists of avoiding carbs at all costs (there's a very low daily limit). If you stay under that limit for about a week, your body becomes carb-starved and starts burning fat. That state of your metabolism is called ketosis, and being in that state is the goal of the diet, so you burn fat more rapidly.
However, if you go over the daily carb limit just one day, the ketosis stops immediately. To go back into ketosis, you need to say under that daily carb limit for about a week again.
Based on all quotes and evidence provided in both the question, answers and comments, I see no reason to exclude the possibility that the wolfsbane effect behaves the same way as ketosis does, i.e. that it requires to be taken for several consecutive days for the effect to activate, but failing to take it for a single day deactivates the effect.
"As long as I take it in the week, preceding the full moon, I keep my mind when I transform."
You're reading this as "I must take it once, a week in advance", but it can also mean "I must take it [daily] for a week".
Note that the frequency (e.g. daily, hourly, ...) isn't specified. I'm assuming it is to be taken on a daily frequency for the sake of keeping things simple, but the answer remains the same regardless of what the exact frequency is.
"He didn’t take his potion tonight! He’s not safe!"
This seems to confirm that not taking the potion immediately (at least within some hours) deactivates the effect.