Saturday Night Live is a weekly live sketch-variety comedy show. The show has been on air since 1975, with differing levels of popularity. For more information on how it's been doing since 1975, check out the History page.
Every week features a different guest host. The host of each episode participates in most of the sketches that are in the show. Hosts are usually chosen by who has a television series out, or who is starring in a new movie. Each episode also features musical guests. Musical guests usually perform twice in each episode, but sometimes, they have an encore performance.
Saturday Night Live broadcasts a usual 20 episodes every season, unless there is an other event going on around the time, or a strike of some sort holds them back from broadcasting. For instance, Saturday Night Live had to broadcast only 19 episodes in season 31, due to the Winter Olympics. Also, during season 6, 13, and 33, there were Writers' strikes, causing them to have only have 13 episodes for both season 6, and 13, and then only 12 episodes for season 33. The only seasons they have ever exceeded the 20 episode line were season 1 (24 episodes), and season 2, and for season 34 (22 episodes).
Since the show is live, that means the cast members, writers, and crew, have to produce episodes the week they air. Monday is idea day, where they pitch ideas, while Tuesday is when they write. Wednesday is the table read, Thursday is blocking day, while shooting promos. Friday is when they run through sketches, and Saturday is when they have rehearsals. They have a rehearsal with costumes at 1 to 5, then they have dinner. At 6, they have a Weekend Update rehearsal, and at 8, they have a dress rehearsal in front of a live studio audience that lasts for an average of 2 hours. When they end anywhere from 9:45 to 10:00, they have a meeting on what material to cut. Once that is done, they have warm-ups, and eventually, they go on live at 11:29 PM, 10:29 central. They take a break off on Sunday, and redo everything starting Monday.
SNL usually takes a few weeks hiatus, in between groups of episodes. Its usual for them to do 2 or 3 episodes in a row, and then take a 1 or 2 week break off, and repeats are usually filled in the timeslot. Once they complete their season in mid-May, they take a summer hiatus, that lasts for a usual 4 month and 1 week. They come back live for their season premiere for the last Saturday of September or the first Saturday of October. On some occasions (season 6, 11, 12, 13, etc.) SNL will start later than that. On other occasions (season 2 and 34), SNL started weeks earlier.