Sorry if this is incorrect, but as far as your answer goes, It should be there on fridays, so something must be wrong. :P
Possible reasoning Lapras isn't showing up:
A: Lapras in found in Union Cave, so if you are searching in, say, Dark Cave, you won't find it.
B: It is in Union Cave's basement, have you checked there?
C: Did you save after beating Morty? (this would be very unfortunate if you haven't)
D: Is your game hacked in any way/are you using an emulator?
If none of these are it, I have no clue what's wrong, but I do know a possible solution. There is another location Lapras can be found, but it is much later in the game (if your interested). In the Safari Zone, specifically in the Rocky Beach Area, there is about a 17% chance of encountering Lapras while surfing.
Safari Zone Source
Hope I helped!