Molly
Greek life is fun and pretty popular at Davis. It is not nearly as popular as it is with a lot of schools in California, though. If you do not join a sorority or fraternity, you will still have no trouble finding a group of friends to hang out with.
At UCD, there are a few different dorm options for incoming freshmen: Segundo, Tercero, Cuarto and Regan. Segundo and Regan will give you the traditional dorm-style rooms down a long hallway. People generally leave their doors open and this is a fun place to live, from what I hear. (Segundo is probably preferable to Regan because the buildings are nicer.) Cuarto dorms are located just off-campus, but are an easy bike ride (or free bus ride) to campus (and if you are a walker, the walk's not so bad either.) Cuarto offers apartment-style living, but without kitchens. Most of the buildings in Cuarto have pools, too. One drawback here is that Cuarto can sometimes be less social than a Segundo type dorm. I had a great experience in Cuarto and loved having a bathroom that was only shared by 4 people, but if I had to do it over again, I would opt for a traditional, Segundo-style dorm. The Tercero dorms are brand new, so I'm not familiar with their style and setup.
We had some really cool guest speakers/performers visit campus while I was a student. (2002-2006.) Bill Clinton, one of the Simpsons' creators/producers, Bright Eyes, the father of a kid shot at Columbine...There were many more but those were the few I attended.
One of the biggest events that happens at UC Davis is Picnic Day. This happens every year on a Saturday in April, and it's like a big huge carnival that takes over the whole town and campus. This event starts with a parade at around 8 in the morning. It includes fashion shows, dauchsund derbies (really cute), dog frisbee catching contests, cool speakers, food, booths galore, bands, and lots more. Many students start drinking when they wake up and don't stop until that night. People look forward to Picnic Day all year. It's a blast.
Every Wednesday afternoon/evening and every Saturday morning, Central Park hosts a farmer's market. Central Park is located a block away from campus and this is one of my favorite things to do in Davis. Local growers bring fresh produce to sell, and students and families come to buy groceries and lay out a blanket on the grass. On Wednesday evenings, particularly in the Spring, students and lots of young parents come and sprawl out while their kids run around and a band plays. Local restaurants set up booths to sell delicious food as well. I highly recommend the Farmer's Market.
Reese
Probably the most popular group on campus is Aggie Pack. Aggie Pack is a student spirit organization focused on promoting Aggie sports. When a freshman goes to his or her first sporting event on campus, they get a free t-shirt and are admitted into Aggie Pack. The Aggie Pack fills the student section, we stand during the whole game, sing the fight song, do traditional cheers, and sing the alma mater afterwards whether we win or lose. All of our sports have just gone to D-1 so the schedules have been a lot tougher lately, but the sporting events are still very popular. We have a bonfire every year before the football season.
Everyone says that the dorm experience is the best and I tend to agree. The RAs plan social and educational events throughout the year to help Freshmen get settled in on campus. Like I said earlier, everyone is really friendly and outgoing.
Parties are a part of life at Davis in that you can find them if you want to. But I wouldn't say that Davis has a "party culture," and Davis is definitely not a party school. It's been my experience that most students party responsibly even when drinking is involved. There are services like Tipsy Taxi that keep kids safe and prevent drunk driving.
Mariele
Certain athletic events(especially football games) are really popular. Students in the dorms do regularly leave their doors open, which contributes to the community feel. There are always different productions and events happening at the Mondavi Center and downtown or in Sacramento, which is nearby.
Matt
Aggie Pack is the largest student spirit organization in the country. CALPIRG is also a popular and highly visible group, recruiting signatures and pledges, and working on behalf of students to lower fees and create other student-friendly legislation. I am a member of the Aggie Recruitment Team, a volunteer outreach group that speaks to prospective UC Davis students about student life. We hold question and answer panels to answer student questions directly, and give them some realistic insight into UC Davis life. Dorm doors are always left open, and people are always welcome. Football is the most popular sport at Davis; the team had a streak of 37 consecutive winning seasons end this year, and attracts a loyal following. All sports programs moved up to Division I as of 2007-08, so they are likely to increase in popularity in the near future, as they are admittedly not a huge draw currently. There are always a handful of events going on, some more popular than others. BB King recently played a sold out show at the Mondavi Center on campus, and Bill Clinton attracted a capacity crowd of over 7,000 to the Pavilion on campus (over 3,000 people had to be turned away). There are lots of quaint restaurants and coffee shops downtown to take a date, and the arboretum on campus provides lots of beautiful scenery to take a date on a walk. I met my closest friends through various classes and events related to my major. If I'm awake at 2am on a Tuesday, I'm listening to my friends' show on the campus radio station, and probably writing a paper. Picnic Day, held in April, is the largest student-run event in the country: it is the open house for UC Davis, which attracts 50,000 to 60,000 visitors. Parades, demonstrations, a fashion show, dachsund races and various other events happen all over campus all day long, and all students and the entire town are invited. Thousands of alumni return to Davis to take part. It is also a heavy drinking day for students, who begin early in the morning and continue into the night, but overall Picnic Day is a very family friendly event. Whole Earth Festival is the second biggest event at UC Davis, an entire weekend in which the university embraces its "hippie" image and promotes environmental awareness and sustainability. The entire Mother's Day weekend involves music and dancing on the quad, which is also lined with arts and crafts booths, and local food. Hackey-sack, hula hoops, tie-dye clothes, shirtless guys and other stereotypically hippie things abound at Whole Earth Festival. In keeping with the "hippie" vibe of Whole Earth Festival, many parties (including one legendary housing location on campus) are havens for drugs. The police seem to look the other way this one day of the year, so if you like smoking marijuana and dancing naked on the roof, feel free. You can find a party pretty much every weekend, and during the week, though not everyone parties this often. Fraternities and sororities are active, but do not exert an overwhelming presence on campus (it's not Animal House). Last Friday I went to a concert/party at my friend's house which was sponsored by the campus radio station. On Saturday I went to a women's/men's doubleheader basketball game. Movies, coffeeshops, music shows at local venues, poetry slams, bonfires, athletic events, and many, many other things are constantly available for entertainment on the weekends. And it is possible to attend the traditional party and not drink--there is little pressure to conform and drink or do any kind of drugs.