"Now is Monday, tomorrow is Tuesday, the day after tomorrow is Wednesday."
Translation:Sekarang hari Senin, besok hari Selasa, lusa hari Rabu.
So apparently the week days shouldn't stand alone. For example: "I went to the market last monday" "Saya pergi ke pasar Senin lalu" This would be allowed since you specified which Monday by lalu/last. If however the Monday stands alone, like in this case you have to add hari.