A Westjet Boeing 737-800, registration C-GWSV performing flight WS-662 from Calgary,AB to Toronto,ON (Canada) with 137 people on board, was in the initial climb when flight attendants reported an odour in the cabin, however, there was no smoke. Climbing through 15,000 feet smoke became visible on the flight deck and cabin. The flight crew donned their oxygen masks and smoke goggles, worked the checklists detecting a left PACK TRIP OFF indication. The air conditioning system was turned off, the crew declared emergency and returned to Calgary for a safe landing about 20 minutes after departure. The Canadian TSB reported that the aircraft had been involved in a similiar event the previous day, see Incident: Westjet B738 at Calgary on Feb 2nd 2017, fumes and smoke in cockpit and cabin, and was performing its first flight after this event. On this occasion maintenance discovered an oil leak in the APU that contaminated the air conditioning system.