I was disappointed to note that the Trade Way Event chart in Chapter 4 of Hoarde of the Dragon Queen listed this event but then seemed to omit it. This isn't the end of the world, unless your quickly getting up to speed because your DnD Encounters group is big enough to split and you have about 15 minutes to prep and then you roll "High Holy Day" and nothing.
So here is what I worked up in a matter of a few seconds:
A heartwarder of Sune, Tyra Silk (Human Cleric, Level 4) is a pilgrim headed to the Radiant Chapel in Waterdeep who asks the Caravan Master to stop a few hours before sunset so the journey can be blessed by Lady Firehair with a celebration of her holy day, the Ruby Fete. Silk is popular amongst the caravan for her flirting, beauty, kindness and the aid she has given throughout the journey. While the Caravan Master is dubious, Silk is able to convince him of the benefit for a good journey and continued morale. Of course, the Fete can easily becomes a raucous affair and is a great time for players to snoop and for the Cult to do so, as well. Its also a good way to introduce some of the NPC's in the chapter.
Tyra is smart, savvy and great with people. As a heartwarder she has taken on several lovers on the journey already and is very aware of what is happening amongst the caravan. She is a good listener and truly cares for the people around her. While not much a combatant, she is a trained healer and gladly uses Sune's gifts to aid anyone she can. The primary reason for her pilgrimage to the Radiant Temple is to inform its' First Mistress of Sune, Chloe Blume, of the attack on Greenest by the Cult of the Dragon, but has no further information to aid the players.