GOT: The reason for the celebration of the Harrenhal Tournament:

This is what we know: the tournament was announced towards the end of the year 280 (A.D.) by Lord Whent, Lord of Harrenhal, not long after the visit of his brother Ser Oswell Whent, member of the Royal Guard. That this event would be magnificent, of which Lord Whent had no doubt, the prizes he offered were three times greater than those offered at the great Lannisport tournament organized by Lord Tywin at 272 (A.D), a tournament that was organized in honor of the ten years that Aerys II was governing on the iron throne.

Most took this simply as a Whent pawn for exceeding the previous hand and demonstrating the wealth and splendor of their home. However, there is only one ruse, and Lord. His lordship lacked the funds to pay such splendid prizes, they argued; certainly someone else behind him, someone with enough gold, but who for some reason prefers to remain in the shadows to leave the glory of the magnificent event to Lord Whent. There is no evidence that any "covert shadows" were behind the organization of the tournament, and there is no such thing today, but that is what they are created in that same year, which some still believe today. But ... what if there was indeed a beneficent shadow behind the entire Harrenhal tournament? Why keep secret? Many names have been proposed for this shadow, but only one seems truly convincing: Rhaegar Targaryen, the prince of Dragonstone.

If you can believe this story, it was Prince Rhaegar, who proposed to Lord Whent to hold the tournament, using as intermediary his little brother Ser Oswell. Rhaegar provided Whent with enough gold to make the prizes even more splendid in order to draw as many great lords and knights to Harrenhal as possible. The purpose of Rhaegar was not to hold a tournament, but to attract the greatest lords of the kingdom to, in an informal form of advice, to deal with the madness of his father, King Aerys II, and probably discuss some form that the king abdicate or leave a regency in his name. If this was really the purpose behind the tournament, it was a dangerous game that Rhaegar was playing. Although very few doubted the dementia of Aerys, still many had good reasons to oppose to remove it from the Iron Throne. Certain courtiers and counselors had gained great wealth and power through the king's whims and knew that they could lose everything if Prince Rhaegar came to power.

The Mad King could be savagely cruel, as was clearly seen when he burned those he perceived as his enemies, but it could also be extravagant: he filled men who pleased him with honors, offices, and lands. The lords surrounding Aerys II had gained much from the madness of the king and enthusiastically took every opportunity to speak evil of Prince Rhaegar and inflame the father's suspicions about the son.