Stardew Valley has 12 festivals a year. This guide lists every date, entry window and main reward, and explains which festivals stop time and which keep shops open, based on the official Stardew Valley Wiki.
Festivals: 12 (Spring 3, Summer 3, Fall 2, Winter 4)
Time keeps passing: Desert Festival, Trout Derby, SquidFest, Night Market
Added in 1.6: Desert Festival, Trout Derby, SquidFest
Source: Stardew Valley Wiki (Korean and English)
Calendar at a glance
Spring 13 Egg Festival - Town Square, 9am-2pm
Spring 15-17 Desert Festival - the Desert, 10am-2am
Spring 24 Flower Dance - Cindersap Forest, 9am-2pm
Summer 11 Luau - the Beach, 9am-2pm
Summer 20-21 Trout Derby - Cindersap Forest river, 6:10am-2am
Summer 28 Dance of the Moonlight Jellies - the Beach, 10pm-12am
Fall 16 Stardew Valley Fair - Pelican Town, 9am-3pm
Fall 27 Spirit's Eve - Pelican Town, 10pm-11:50pm
Winter 8 Festival of Ice - Cindersap Forest, 9am-2pm
Winter 12-13 SquidFest - the Beach, 6:10am-2am
Winter 15-17 Night Market - the Beach, 5pm-2am
Winter 25 Feast of the Winter Star - Pelican Town, 9am-2pm
The four festivals where time keeps passing
In most festivals the clock stops and you are sent back to the farm around 10pm when it ends. If a festival starts before 7pm, homes and shops are locked all day, even if you skip the event (the Sewers, Community Center and Spa stay open). The Desert Festival, Trout Derby, SquidFest and Night Market are the exceptions: time keeps passing and shops and homes stay open. For those four you must feed your animals yourself; for every other festival the game counts them as fed.
Spring and summer rewards
Egg Festival: talk to Mayor Lewis to start, then collect 9 colored eggs in about 50 seconds of real time. Your first win gives a Straw Hat; later wins give a Prize Ticket. The booth sells Strawberry Seeds, which can give two harvests before summer if planted by Spring 16.
Desert Festival: three days of rotating shops, a race to bet on, challenges for event currency and a daily chance at a new outfit from Emily.
Flower Dance: ask a bachelor or bachelorette to dance by talking to them twice. They refuse below four hearts, and dancing adds one heart (250 points).
Luau: the ingredient you add to the potluck soup, and its quality, changes how the Governor reacts and can raise or lower friendship with every villager.
Trout Derby: catch Rainbow Trout all day; some carry a Golden Tag that you trade for prizes such as the Tent Kit, Bucket Hat and Mounted Trout.
Dance of the Moonlight Jellies: no shop. Talk to Lewis to begin and watch the jellyfish; it ends on its own.
Fall and winter rewards
Stardew Valley Fair: earn Star Tokens from minigames and the Grange Display (up to 9 items) and spend them at the shop. Items are not returned automatically; if you forget, check the lost and found box in the Mayor's Manor.
Spirit's Eve: the maze ends in a cave with a Golden Pumpkin, and Pierre's festival shop sells rare scarecrows and Jack-O-Lanterns.
Festival of Ice: an ice fishing contest with Pam, Willy and Elliott; catch at least five fish to win. The first win gives fishing tackle and a Sailor's Cap, later wins give a Prize Ticket.
SquidFest: for every reward you need 8 Squid on the 12th and 10 Squid on the 13th; the counter resets at the start of the 13th.