Enter the temple of light and approach the small alter in front of the monk at the back of the room. Use it to make a donation to the temple. Give them about 1,000 gold. The monk will then tell you that if you donate a large sum of money when the Sun reaches its zenith, you may be rewarded.

Check your pause screen to track the time, and when it reaches somewhere between 12:00pm and 1:00pm, use the altar once again and donate around 10,000 gold pieces for your big reward.

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Interact with the altar (left) and donate around 10,000 gold between 12pm and 1pm.