Thaipusam is a Hindu holiday, this year celebrated 2-3 February. For an official explanation of this holiday, please read this Wikipedia article. For my very limited understanding, please read on. This holiday celebrates the victory of Lord Murugan over the devil. Lord Murugan is glorified by the giant gold statue at the foot of Batu Caves. I feel pretty confident that what I’ve said so far is accurate. I feel less confident about the following because I don’t have a very in depth knowledge of the Hindu belief system, so I’m sure I will get some of the specifics wrong. My apologies in advance and I would welcome any corrections and opportunities to learn in the comments!
Devotees pray to Murugan for his help to overcome some obstacle in life. When Murugan helps you through this obstacle, you celebrate him and make an offering to him on Thaipusam. It reminds me of Catholics celebrating a particular Saint’s feast day, actually. (But I’m not Catholic, so what do I really know about that, either.) Devotees cleanse themselves through prayer and fasting before making their offering. One traditional offering is a milk pot. You carry your milk pot (on your head) up the steps at Batu Caves- there are 272 steps so it is not a simple journey. When you reach the temple at the top of the steps, a priest gives your offering to Murugan. (And I guess you get your milk pot back? I’m not sure. I don’t remember seeing either a pile of milk pots or people carrying empty ones back down either.) The shrine ends up looking like a giant pool of milk. We saw people carrying small-ish containers that I assume hold between a half-gallon and a gallon and people carrying much larger containers. We also saw one family offer a pint of milk, still in the plastic bottle that you can buy at the corner store. I think the vehicle likely matters less than your intent.
If you want to make an even larger offering, you can build and carry a structure that I learned is called a kavadi, which means a physical burden. It might be relatively small, and you can carry it over your shoulders without much of a support crew. Or it might be several meters tall and require a shoulder rig and several people to help you. The kavadi were very elaborate and often brightly colored with flowers and peacock feathers.
You might also choose to pay tribute to Lord Murugan by practicing flesh mortification. I believe most of the kavadi bearers did this, but there were others not carrying a kavadi who practiced this as well. Maybe you will pierce your face with a large steel bar. Maybe you will pierce the skin on your chest or back with fish hooks and hang chains or other small objects from them. Maybe you will do something else. If you choose to go this route, a holy man will remove the piercings once you reach the top of the stairs. (There are also medics nearby should you need them. The Wikipedia article says that no blood is shed either during the piercing or the removal.)

I actually don’t know what this is, but someone was pushing it in a wagon and it was pretty cool looking.
This all sounds very dark but there was definitely a festive atmosphere. There were emcees leading chants and playing music, as well as perhaps offering narration or prayers- it wasn’t in English, so I’m not sure. The support crews for the kavadi-bearers often included people playing drums or other percussion instruments, singing and chanting. Many people carrying the kavadi danced while they waited their turn to ascend the steps. The plazas at the base of the steps were also filled with food and drink vendors.
We chose to experience this holiday and festival as tourists. (We didn’t give an offering, but we did donate monetarily to the temple.) We did walk up the stairs in a giant crush of people, and back down again in the same giant crush of people. It took several hours, where as it might take one hour or so on a non-holiday. Someone asked me at the hotel afterwards if I had a good time. I don’t think this is really a “good time” kind of thing, but I appreciated witnessing the festival and having a cultural experience that I will likely never have the opportunity to experience again. What made the biggest impression on me was seeing the variety of ways that people chose to express their faith and devotion as they made offerings to Murugan.
In addition to the pictures included here, there are more on our Flickr page along with some more in-depth captions. Please be aware that many of those pictures include devotees practicing flesh mortification. There’s no blood or gore, but it might not be your cup of tea and I thought you should know that before you clicked on the link. (Not all of the pictures are of devotees and flesh mortification, but there are some.)