Okay, people really do need to learn that fandom wikia has done a mistake about the Wolfsbane potion that completely changes the interpretation of Lupin’s negligence in PoA.
Fandom wikia says that Lupin has to take Wolfsbane every day during the week preceding the full moon. It’s bullshit. I’ll show you why:
"I made an entire cauldronful,” Snape continued. “ If you need more."
—> Wolfsbane has to be drunk in sufficient amounts.
“I should probably have some again tomorrow."
—> You don’t have to take the potion on a rigid 7 days schedule, seeing as Lupin days he might need probably more the next day.
"The potion that Professor Snape has been making for me is a very recent discovery. It makes me safe, you see. As long as I take it in the week preceding the full moon, I keep my mind when I transform...
—> Notice Lupin says "as long as I take it "in" the week preceding the full moon, and not "during" the week, proving again you only need to take it once for the Wolfsbane to work.
Final fact? Nowhere is it said in the books that Lupin has to take it seven days in a row, and all the evidence that we get rather points to the contrary, for the objective eye.
TLDR: Lupin only has to drink Wolfsbane at least once in the week preceding the full moon, in sufficient amounts.
When he transformed at the end of the year in PoA, he had neglected drinking the Wolfsbane for seven days already. The night he was supposed to take it, he was reading the Map in his office, instead of moving his ass to go fetch it. It had to be Snape—again—to bring him his own medication like a servant. That’s why Lupin is so at fault in PoA.