My neighbor only smoking cigarettes on Sundays is marbles

Every Sunday night, without fail, I sit in my apartment and smell cigarette smoke. I know it's coming from my neighbor, but I can't figure out why he only smokes on Sundays. He's there all the damn time (I know because I can hear him), but apparently on Sunday nights he either decides to get in some work on his lung cancer, or he invites over his chain-smoking grandmother who is evidently too feeble to step outside to light up and too addicted to just hold out for a few hours. Because there's no way any sane person would allow anyone else to just sit in his apartment and smoke all night, or would do the same himself on only one day of the week.