King of Avalon almost always has one or two seasonal events being active. At this time the Spring Festival Event is one of them. This guide explains how the event works.
During the duration of the event you get a chance of receiving Flare Powder when you gather resources and Parchment when defeating Monsters or Barbarian Camps. You can Synthesize 8 Flare Powders and 1 Parchment into a Firecracker. The Firecracker you can use to summon a Nian Beast on the map. The Nian Beast will appear on the map at a spot specified by you and comes with a timer of 60 minutes. You and your alliance members should rally it and if you are able to do damage corresponding to 100% of the Hit Points everyone on the alliance will receive a Red Envelope.
It is relatively easy to beat the Nian Beast, but of course you need to make sure there are some other team mates online to rally with you.
A lot of players tend to want to set their own rallies when they summon the Nian Beast, which is nice, but in the end you also want to consider who has the highest level Hall of War.
As typical for these type of events in King of Avalon, it is about doing damage, so don't go for the one perfect rally, but rather have many rallies. You don't need to worry about losing a rally.
Since defense is not important for these type of events, I tend to go heavy on damage dealers like Bowmen and Cavalry, and low on Infantry. I have not experimented much with Siege yet.
The Red Envelopes can contain all kind of small useful items, like Gold, Wood, Food, Lord EXP, Dragon Spirit EXP, VIP Activation, VIP Points, Steel, Rune Stone, Speed Ups, Materials (Animal Hide, Cerussite, Pine, Bronze), Farm/Sawmill Boosts. One envelope doesn't seem like much, but if you are in a large alliance where everybody is actively gathering and slaying monsters, it could be that you beat dozens of the Nian Beast every day, and in that case the prizes really add up.
Besides the Red Envelope, the players participating in rallying the Nian Beast also get 1k Food and Wood for every percentage damage they do.
As other seasonal events the Spring Festival Event is a nice way to get some extra goodies.