There are many annual festivals held in Harvest Moon DS. Here is a description of all of the events.
1st: New Years Day Festival
This event takes place at 6 PM. People from the valley gather around to eat rice cakes. When the festival is over, if you had a spot open in your rucksack, then Thomas will say he made too much and offer to give you some. When you go home, you will have 10 rice cakes.
14th: Spring Thanksgiving Festival
This festival takes place at your farm and lasts all day. You do all your normal chores and give a cake to the girl that you are in the process of courting. If she really likes you, then you will trigger a cut-scene.
18th: Duck Festival
The duck festival takes place from 10:00 AM to 6:00 PM at the beach. To enter your duck, you need to give it to Popuri when she comes the day before, and it must not be sick. If you win, your duck lays golden eggs when it reaches 10 hearts.
22nd: Cooking Festival
This, for those who own FoMT, is essentially the same. Thomas will announce the day before that there is to be a cooking festival. You go to the beach some time after 10:00 AM, and talk to Gourmet and Thomas. You will then be back in your house, ready to cook. You can try to enter if you don't have any utensils, but it helps to have them. You then show your creation to Thomas, and he'll enter it into the competition. No matter what, you will always be the last one to be judged. You can tell if Gourmet likes the food or not by the letters he makes above his head. He'll either spell G.O.O.D, or B.A.D. If it's not part of the theme for food, then he'll say it's rather tasty, but you're disqualified.
1st: Beach Festival
This takes place at the beach between 10:00 AM and 6:00 PM. It is a welcome for Kia, as well as a festival to mark the beginning of Summer. You can go talk to everyone and goof off. When you feel like ending the festival, go and talk to Kia, he will give you some free food, and the festival is finished.
7th: Chicken Festival
This is also at the beach, and as usual, between 10:00 AM and 6:00 PM. Popuri shows up a day before, takes a chicken, and goes off with it. If you go to the festivities the next day and talk to Popuri, she'll judge the chicken, and the festival is over. If you win, hatch the eggs that the chicken produces, and they will hatch into level 2 chickens.
20th: Cow Festival
The cow festival is also at the beach. Rick will come and pick up the cow the day before, and you go to the beach and speak to him in order to get the festival started. They bring out all the cows, one by one, and then, at the end, the winner is announced. If you win, your cow goes up a level, and impregnating this cow makes higher-levelled calves.
24th: Fireworks Festival
The fireworks event is from 6:00 PM until 12:00 AM. This is the event where you get to hang out with the girl that you are courting. If any eligible brides are at a green heart or more and you speak to them, they will ask you if you would like to watch the fireworks with them. If not, you'll need to talk to everyone there before the fireworks begin. If you do go with your girl, her affection towards you will go up.
9th: Harvest Festival
The Harvest Festival starts at 10:00 AM and goes until 6:00 PM. The point of this festival is to have the whole town come together and make a pot of stew out of their own earnings. The people will tell you how much they have been looking forward to this day. After you put your delectable (or despicable) item into the pot, the festivities begin. It shows a simple cut-scene of you eating a bowl of the stew, and then you passing out or saying G.O.O.D. When you talk to someone else, they'll either say it was good or bad, depending on the ingredients.
20th: Sheep Festival
As with all the other animals, Rick comes, takes the sheep of your choice, they judge it, and you're done. The same still applies with winning. If you win, your animal will increase its affection towards you. The animal, and its babies afterwards, will also level up.
30th: Pumpkin Festival
This has absolutely nothing to do with pumpkins. It is Harvest Moon's version of Halloween. Kids, and some adults, come to your house in the morning, asking for candy, or more specifically, chocolate. They won't leave for a while, unless you give them something. However, it is quite humorous to watch all the children get angry when you give them boiled eggs instead…
14th: Winter Thanksgiving Festival
Just like in the Spring, this involves cake. Except this time around, you get to receive the cake. If you are married, then your wife will slip it into your rucksack for you when you wake up. If you are dating, your girlfriend will come and drop it off sometime in the day. You will need to enter and exit your house in order to trigger it. However, if a friend of your girlfriend is sent to deliver the cake for her, then she is ready to be married.
24th: Starry Night Festival
This takes place on the beach between 6:00 PM and 12:00 AM. If you have a girlfriend, then she will attend the festival with you. You do little other than look at the stars.
25th: Stocking Festival
This festival does not even require you to be awake, so go to sleep, hobo Jack. Thomas sneaks into your house and puts a present in your stocking. However, if you don't have a stocking, he will just say "Oh well" and leave. In order to get the stocking, you must befriend Nina the old-fashioned way. Give her 50 large wool, and some gifts. Don't use the dog or cat trick. That does not cut it. You will then walk in on a sunny afternoon, and she will give you a stocking.
30th: New Years Festival
This takes place from 6:00 PM until 12:00 AM. You go to the beach and chat about all the things you have accomplished; you then chow down on some buckwheat noodles. If you have the free space, Thomas will give you some leftovers. There is a 50% chance that it will be the noodles, and another 50% chance that it will be the flour itself. You then go home and wake up, ready to celebrate New Years Day Festival.