Yes, you can buy ticket when you arrive int he park. It is possible to see them in one day. Get up early to take mtr to Tung Chung first. Waiting for the Nging Ping 360 cable car （9am in weekends and holiday, 10am in ordinary days). Buy round trip ticket. There maybe waiting line to but ticket and take the car, suggest you get there earlier, because you still have save time for disneyland. Get off the cable car, you can see Nging Ping bazaar and then walk 5 minutes to the BIg Buddha. Get back to Tung Chung, and then take mtr to Sunny Bay. Shift to Disney Special Line to the park. Stay at Disneyland till night. The firework show is at night.