At 8:00 PM your in-game time, you receive a daily reward based on your Grand Arena ranking at the time.
This reward arrives in a letter in your in-game mailbox. The game checks positions and assigns rewards exactly at 8:00 PM, though it might take a few minutes for the reward to reach you. Even if your position changes a second after 8:00 PM, it will not affect your reward.
To receive the daily reward for your ranking, you need to attack at least once in the Grand Arena before 8:00 PM, your in-game time zone. You can check the time zone by tapping your avatar in the upper-left corner. Note that successful defenses are not taken into account when issuing the reward.
Besides, you receive 10 Grand Arena Coins only for the first five successful defenses, up to 50 Coins per day. All further defenses are not rewarded. The Coins arrive in a letter in your in-game mailbox.