From the 2016 Christmas Update on, there have been a continuous series of events in which players can train troops or brew spells at reduced cost for a limited time, although there were also several Events before that Update which didn't reward gems. The troop events function like mini-achievements, in which players need to win a certain number of multiplayer battles, each with a certain number of the said troops, to claim some Experience and Gems. Most Events' discount rates are 90% (i.e. reduced to 10% of normal cost).
Using the exact number of the said troops is not required; it's totally OK to use more than the number of the required troops, as long as at least one star is achieved. The troops in the Clan Castle also count for the number of troops, and are essential for players with lower Town Hall levels that can't train the said troops by themselves. If the Town Hall level is low enough such that neither training nor requesting the said troop is possible, a number will still be displayed, but the mini-achievement reward is unattainable unless you rush the building. Also, players can aim for this reward other in-game tasks simultaneously.
For multi-troop events, the reward for each troop is achieved separately, but players can aim for them simultaneously. The requirement for each individual troop is equal to the requirement of that troop's single troop event.
Below is a table about how many troops are needed in each type of event. For most troop events, players need to have at least one fifth of total Army Camp space possible at the Town Hall level being that troop, the number of troops required are rounded to the nearest integer. If the Town Hall level is not enough to unlock the said troop, the requirement becomes filling as many as possible to the maximum level of Clan Castle at the Town Hall level.
The Town Hall level might be the sole factor of the troop requirement, regardless of whether or not the Barracks or Dark Barracks (to unlock the said troop), Army Camps and the Clan Castle are at the maximum level possible at the TH level. For example:
A new TH 9 with two level 4 Dark Barracks still requires 4 Witches for the Witch Event, and is therefore locked outside of the reward without using Gems or Magic Items to upgrade the Dark Barracks to level 5. Had his Town Hall's upgrade not finished during the event, he would have been able to claim the reward by using two Witches as Clan Castle troops. Same goes for the Witch part of a multi-troop event.
A new TH 10 with four level 7 Army Camps requires 10 Balloons rather than 9 for the Balloon Event.
In all these events, players get a chance to gain some resources by queuing just before the end of the event and unqueuing after the event ends. Theoretically, an Event of 90% discount on Electro Dragons and Jump Spells will yield the maximum amount of Elixir possible (964,800 ) and an Event of 90% discount on Bowlers and Earthquake Spells will yield the maximum amount of Dark Elixir possible (18,561 ). For multi-troop events, players can also choose to get a mix of both resources.
The tables below are the amount of resources obtainable this way at each Town Hall level, assuming a 90% discount.
During each annual anniversary of the game (late July to early August), players are able to boost each Gold Mine, Elixir Collector and Dark Elixir Drill for 7 days with 1 gem. Normally, each player can boost each of his/her collectors for one round, but if they're not at the maximum level, he/she can choose to press upgrade and cancel and boost for a second round, but will waste half of the upgrade cost (especially for Dark Elixir Drills, which have high upgrade costs).
However, in the 2017 Clashiversary event, one gem only boosted for 1 day instead of 7 days.
The 2018 Clashiversary event still has one gem boost for only 1 day, but the event will last 31 days (for the whole month of August).
During each Christmas, players are able to boost each Gold Mine, Elixir Collector and Dark Elixir Drill for 1 day with 1 gem, with the maximum number of rounds equal to the number of days in the event.
In the days immediately following the release of Town Hall level 12, players are able to boost all resource collectors for 1 day with 1 gem; this started at UTC 13:00 11/6/18 and was initially scheduled to last 7 days; following a maintenance break on 15/6/18, this event was extended by another 7 days.
One-Gem Troop Training Boost Events
During each Christmas, players are able to boost each Barracks, Dark Barracks and Hero with 1 gem. However, the gem boost "discount rates" vary in each year's event. A 7-day Valentine Hero boost event also started on UTC 13:00 13/2/17.
In the days immediately following the release of Town Hall level 12, players are able to boost all barracks and heroes for 1 hour with 1 gem; this started at UTC 13:00 11/6/18 and was initially scheduled to last 7 days; following a maintenance break on 15/6/18, this event was extended by another 7 days.
One-Gem Spell Brewing Boost Events
During each Halloween, players are able to boost the Spell Factory and the Dark Spell Factory with 1 gem. However, the gem boost "discount rates" vary in each year's event.
In the days immediately following the release of Town Hall level 12, players are able to boost both Spell Factories for 1 hour with 1 gem; this started at UTC 13:00 11/6/18 and was initially scheduled to last 7 days; following a maintenance break on 15/6/18, this event was extended by another 7 days.
One-Gem Donation Events
Started
Days
11/6/18
7+7*
3/2/19
8
*Originally meant to last 7 days, but was extended by another 7 more days following a maintenance break.
During these events, the gem cost to instantly donate troops, spells and Siege Machines was reduced to one.
Increased Clan XP and/or resources can be awarded if any part of a war lies in the interval of an event.
The first three Events were celebrating the first through third anniversaries of Clan Wars, respectively. If a Clan XP and/or War Bonus Resources Event lasts for n days, players are able to get a maximum of $ \Bigg\lceil{24n \over 47}\Bigg\rceil+1 $ wars with increased rewards during this event. The first event occurred before preparation lengths were shortened (during late 2015); the maximum number of wars with increased rewards during that event was 5.
During the Star Bonus events, the Star Bonuses are multiplied by a factor. If a Star Bonus Event lasts for n days, players are able to get a maximum of n+1 increased Star Bonuses during this event, probably having to waste some "Star Bonus Time", but still beneficial to do so.
During these events, players get Win Bonuses through Versus Battles every 8 hours instead of every 22 hours.
During the second and third event, players' Win Bonuses were also increased by 25% (the third Win Bonus has the same amount of loot increase, rather than percentage increase, as the first two wins).
During this event, Barbarians and Dragons trained 5 times quicker; the Barbarian's training time was reduced from 20s to 4s while the Dragon's training time (at the time) was reduced from 30m to 6m.
Hammer Jam Season
Started
Days
28/5/18
14
During this event, building and hero construction/upgrade times in both villages are reduced by half. Since this event reduces time rather than making the countdown twice as fast, this means: stacking with Builder Potion and Clock Tower boosts multiplicatively; fewer experience points are gained from constructions/upgrades; players can start constructions/upgrades when the event is almost over to save more time and still gain normal amount of experience.
Building/Hero Destruction Events
Destroy a certain number of Buildings, or knock out a certain number of Heroes in Raids or Versus Battles. Note that unlike in Clan Games and Season Challenges, Heroes are counted by number rather than level.
It was the second of the 2017 Clashiversary events. This event, while adding no new content by itself, temporarily changed the mechanics of the Builder's Hut to spawn a level 1 Giant whenever it was destroyed.
It was the third of the 2017 Clashiversary events. The event allowed players to place the Shrink Trap in their Home Village temporarily. See Shrink Trap for more information.
) and an Event of 90% discount on Bowlers and Earthquake Spells will yield the maximum amount of Dark Elixir possible (18,561 
). For multi-troop events, players can also choose to get a mix of both resources.
and 5 
