The gathering at Minneriya is such an amazing sight. It is a must see if you are in Sri Lanka around July-September. I saw about 300 elephants and lots of baby elephants. It's good to enter the park at about 3pm because elephants come out of the thick jungle to the water tank late evening, so the later you go the more chance to see lots of elephants. I think the ticket counter close at 5:30pm. However if you are by the tank around 4pm this should be fine!