Halloween season

File:GhostCookies.png

Halloween Season is a seasonal event in Cookie Clicker, that was added in the 1.039 update.

The event was on during versions 1.039 - 1.0403 without a way to turn it off.

The seasonal event upgrade was added in version 1.041.

Since 1.0466 update Halloween season starts automatically and lasts from 24th to 31st of October (from 23rd to 30th of October for leap years).

Halloween season will be activated by purchasing the Upgrade "Ghostly biscuit", which will launch it for 24 hours.

The upgrade will be available after unlocking the Season switcher, which costs 1,111 Heavenly chips and baking at least 10 trillion cookies baked in the current game, that let you trigger seasonal events at will, for a price. 

The upgrade is repeatable, but will get more expensive every time you buy it, like Elder Pledge. It will also cancel any other seasonal event that is on at the time you click it.

When purchased, it will unlock the features of the Halloween season: Halloween-related cookie upgrades, and Halloween Wrath Cookies (Also known as "Scary Cookies").

Halloween-related cookie upgrades are only available by popping Wrinklers, which means that at least the One Mind upgrade has to have been purchased and the Elder Pledge is not in effect.

The appearance of Wrath cookies during the Halloween Day season:

Upgrades

List of the 7 known cookie Upgrades that will be dropped from Wrinklers in Halloween season. Each cookie costs 444,444,444,444 (444.444 billion) cookies.

Icon	Name	Unlocked by	Description	ID #
File:SkullCookies.png	Skull cookies	$1-f$chance from popping a Wrinkler. See below to know how to calculate $f$.	Cookie production multiplier +2%. "Wanna know something spooky? You've got one of these inside your head RIGHT NOW."	134
File:GhostCookies.png	Ghost cookies		Cookie production multiplier +2%. "They're something strange, but they look pretty good!"	135
File:BatCookies.png	Bat cookies		Cookie production multiplier +2%. "The cookies this town deserves."	136
File:SlimeCookies.png	Slime cookies		Cookie production multiplier +2%. "The incredible melting cookies!"	137
File:PumpkinCookies.png	Pumpkin cookies		Cookie production multiplier +2%. "Not even pumpkin-flavored. Tastes like glazing. Yeugh."	138
File:EyeballCookies.png	Eyeball cookies		Cookie production multiplier +2%. "When you stare into the cookie, the cookie stares back at you."	139
File:SpiderCookies.png	Spider cookies		Cookie production multiplier +2%. "You found the recipe on the web. They do whatever a cookie can."	140

The basic fail rate

$f$ is 0.95. There are many factors that can reduce the fail rate.

	factor
Spooky Cookies	0.8
Santa's Bottomless Bag	0.9
Mind Over Matter	0.75
Starterror	0.9
Shiny Wrinkler	0.9
Selebrak, Spirit of Festivities	0.9/0.95/0.97 (Diamond/Ruby/Jade)

For example, if you popped a wrinkler with the Spooky Cookies achievement and the Santa upgrade Santa's Bottomless Bag, then the fail rate will be

$f=0.95*0.8*0.9=0.684$ . If you popped a shiny wrinkler with all other four non-spirit factors, the the fail rate will be

$f=0.95*0.8*0.9*0.75*0.9*0.9=0.41553$ .

Cookie Spawn Probabilities

Upgrade Probability Distribution

Exploding a Wrinkler after it has begun to feed on the big cookie has a 5% chance of unlocking 1 of the 7 Halloween-themed cookie upgrades. If you have the Spooky cookies achievement, it increases to 24% chance. However, if the particular upgrade chosen at random is already unlocked, it will not unlock a new one. So on each Wrinkler explosion, the actual chance of unlocking a new cookie type is equal to

$s \cdot \left(1-\frac{N}{7}\right)$ , where N is the number of upgrades already unlocked and

$s=1-f$ is the success rate (0.05 normally, 0.24 with "Spooky cookies", 0.145 with "Santa's bottomless bag", and 0.316 with both). The table below shows how the success rate

$r$ effects the expected value of Wrinkler explosions needed to unlock all 7 Halloween cookie upgrades. For example, with bare success rate

$s=0.05$ , the expectation value is

${\frac {18.15}{0.05}}=363$ , however with "Santa's bottomless bag" and "Spooky cookies" the expectation value drastically reduces to

${\frac {18.15}{0.316}}\approx 57$ .

Probabilities for Halloween-Themed Cookies

Cookies Unlocked	Probability of Appearance
	Formula	Base (s=0.05)			With Spooky cookies (s=0.24)			With Santa's bottomless bag (s=0.145)			With Cookies & bag (s=0.316)			With all upgrades & achievements (s=0.5383)
0	s*7/7	5%		1/20	24%		1/4.2	14.5%		1/6.9	31.6%		1/3.2	53.8%		1/1.9
1	s*6/7	4.29%		1/23	20.6%		1/4.9	12.4%		1/8.0	27.1%		1/3.7	46.1%		1/2.2
2	s*5/7	3.57%		1/28	17.1%		1/5.8	10.4%		1/9.7	22.6%		1/4.4	38.5%		1/2.6
3	s*4/7	2.86%		1/35	13.7%		1/7.3	8.29%		1/12	18.1%		1/5.5	30.8%		1/3.3
4	s*3/7	2.14%		1/47	10.3%		1/9.7	6.21%		1/16	13.5%		1/7.4	23.1%		1/4.3
5	s*2/7	1.43%		1/70	6.86%		1/15	4.14%		1/24	9.03%		1/11	15.4%		1/6.5
6	s*1/7	0.714%		1/140	3.43%		1/29	2.07%		1/48	4.51%		1/22	7.69%		1/13
Expected value for Wrinkler explosions to unlock all	18.15/s	363			76			125			57			34

A method to get all the cookie upgrades quickly is to wait for a few wrinklers to spawn, wait for an autosave ("Game Saved" popup), pop the wrinklers quickly, and - if no cookie upgrade appeared - quickly hit F5 to reload before the game is autosaved again - the wrinklers will be back and you can try popping them again.

Another way, similiar to the first, is to open mulitple tabs with wrinklers, and pop the wrinklers on each tab until you get a cookie upgrade. Always leave one tab open with wrinklers, so if all the other tabs fail to give you a cookie upgrade, you can manually save on the tab that still has the wrinklers, and repeat the process.

Achievements

List of known Achievements that will be unlocked eventually from the Halloween season.

Icon	Name	Description	Number
File:SkullCookies.png	Spooky cookies	Unlock every Halloween-themed cookie. Owning this achievement makes Halloween-themed cookies drop more frequently in future playthroughs.	89

Trivia

• The Halloween Season was first confirmed on Orteil's tumblr.
• The Ghost cookies' text references the Ghostbusters theme song.
• The Bat cookies' text references the movie Dark Knight.
• The Eyeball cookies' text references a Friedrich Nietzsche quote.
• The Spider cookies' text references the Spider-Man cartoon theme song, as well as containing a joke about the internet.
• The price of the cookies being four sets of fours because in some asian languages (e.g., mandarin chinese, japanese) the number 4 sounds like the word for death.