Dude, Where's My Car?

Jesse and Chester awaken with hangovers and no memory of the previous night. The television is on, showing an Animal Planet program about how animals use twigs and rocks as tools to get food. Their refrigerator is filled with containers of chocolate pudding, and the answering machine contains an angry message from their twin girlfriends Wilma and Wanda as to their whereabouts. They emerge from their home to find Jesse's car missing, and with it their girlfriends' one-year anniversary presents. This prompts Jesse to ask the film's titular question: "Dude, where's my car?"
Because the girls have promised them a "special treat", which Jesse and Chester take to mean sex, the men are desperate to retrieve their car. The duo begins retracing their steps in an attempt to discover where they left the car. Along the way, they encounter a transgender stripper, a belligerent speaker box operator at a Chinese restaurant's drive-through, discover two appropriately-worded tattoos on each other's backs, run into UFO cultists led by Zoltan, a Cantonese-speaking Chinese tailor, the Zen-minded Nelson and his cannabis-loving dog, the aggressive jock Tommy and his friends, a couple of hard-nosed police detectives, and a reclusive French ostrich farmer. They also meet two groups of aliens, one group being five gorgeous women, the other being two Norwegian men, searching for the "Continuum Transfunctioner"; a mysterious and powerful device, capable of destroying the universe, that the boys accidentally picked up last night.
In an arcade, they discover that the Continuum Transfunctioner was a Rubik's Cube that Chester has been working hard to solve, and eventually does (thus activating it). They are warned that once the five lights stop flashing, the universe will be destroyed.
Jesse and Chester must determine which of two sets of aliens is entitled to the device. One of the groups is there to protect the universe, the other is there to destroy it. Both claim to be the protectors of the universe, stating that they were with Jesse and Chester the previous night (which Jesse and Chester still cannot remember) and ask for the Transfunctioner. The two correctly choose the men, because when the men were asked what they did the night before, they correctly respond that they got a hole in one at the 18th hole at a miniature golf park, and won a lifetime supply of pudding. At the last second, they deactivate the Transfunctioner, saving the universe.
Balked, the five alien women merge to become a giantess played by Jodi Ann Paterson who swallows Tommy alive. The giantess then crawls out of the amusement center and chases Jesse and Chester. The cultists tell them to activate the Photon Accelerator Annihilation Beam on the Transfunctioner. However, the button that activates it is too far in to reach. At the last second, Chester remembers the nature show with the tool-using chimps and uses a straw to push the recessed button, thus destroying the alien. The protectors erase everyone's minds concerning the events and time is reversed to the beginning of the film. The duo recover the car, a Renault Le Car, which turned out to be behind a Mail Truck the whole time, and salvage their relationships and discover the special treat from the girls turns out to be matching knitted caps and scarves. The protectors leave a gift for their girlfriends (and, for the two men): Breast Enhancement Necklaces.

Question: What anniversary are they trying to celebrate for their girlfriends?
Answer: one year