Lapras's appearance is not tied to the day of the week. Instead, it appears in the following locations:
* Route 40: Lapras can be found in the water at the bottom of the hill (near the Safari Zone) with a 1% chance of appearing.
* Route 45: Lapras can be found in the water at the bottom of the hill (near the entrance to the Ice Path) with a 1% chance of appearing.
Important Note: Lapras has a much higher chance of appearing in the water at the bottom of the hill on Route 45.
Remember that you'll need the "Good Rod" to have a chance of encountering Lapras. This rod is obtained from the old fisherman in the Goldenrod City Department Store after beating the Elite Four and becoming the Champion.
Good luck finding Lapras!