This is probably my definitive list of the things I love to do in and around Guadalajara. My top five from this list are Holy Week, the Dance of the Tastoanes, Independence Day, La Romeria, and Dia de los Muertos.
January- If you're thinking of visiting one of the hotter parts of the country, this is the time to do it and it's a little quieter after the Christmas rush and before spring break. I also recommend going to Cajititlan for Dia de los Reyes Magos.
February- Eat tamales for Candlemas. It's still a pretty good time to go to the hot parts of the country. Lent usually starts in February with Ash Wednesday. There's not really a Carnival celebration in Guadalajara, but there's supposed to be something inChapala, Ameca (about an hour or so southwest of GDL), Sayula (almost two hours south of GDL near Ciudad Guzman), Autlán (about 3 hours SW on 80), Jalostotitlán (near San Juan de los Lagos), or Amatitán (the other Tequila town)
the weekend before Ash Wednesday. Capirotada is a popular thing to eat that you can only get at Lent.
March- Benito Juarez's birthday gives you a long weekend the third weekend of this month. The coast can be insane (and the traffic getting there and back), but if you head inland you'll find it's not so crowded. We went to Guanajuato one year and it was perfect. Also, Holy Week is often in late March. Thursday of Holy Week is a good time to visit churches because nearly all of them will be open, including some that are only open that day. Also, you want to eat the pastries that are sold everywhere for Holy Week. And Holy Week can be a great time to visit Mexico City because everyone leaves town (Guadalajara is quieter too). And it's cooler in Mexico City than Guadalajara.
April- Holy Week if it wasn't in March. This includes Palm Sunday processions at pretty much any church (we went to the Basilica of Zapopan), Holy Thursday at every church (the tradition is to visit seven churches that day and I recommend heading downtown because there are lot of churches there and many people visiting them), Via Crucis processions at every church on Friday (the Basilica of Zapopan is good), and Easter Vigil on Saturday evening. April and May are really the only months I don't much like being in Guadalajara because it's so hot so we haven't done a lot these months.
May- Wait for the rain to start. Holy Cross Day is May 3rd but despite trying, we never found any place to go that's worth recommending here. Everything is really dry by this point too so this isn't my favorite time to visit archaeological sites either. It's usually a little cooler at Lake Chapala.
June- Dance in the rain. This can be a good time to go to Tlaquepaque, particularly at the end of the month.
July- Tastoanes in Tonala on July 25th with a women's Tastoanes the following Friday evening. You can also see the Dance of the Tastoanes on July 25-27 in San Juan de Ocotan, Nextipac, and/or Ixcatan.
August- Ascension is on the 15th and this is a huge day in San Juan de los Lagos. So huge that I can't recommend going, but it would be quite the experience.
September- Tastoanes in Jocotan (at the church on Lopez Cotilla between Pedro Loza and Zaragoza) on around September 7th and in Santa Ana (near Bugumbilias) on around the 9th. There's also a mariachi festival at the beginning of the month. The biggest thing is Independence Day on the 16th and it's a great day to be out on the streets. We highly recommend going to the Basilica of Zapopan or to the Cathedral for the Grito and fireworks and light shows at night. This is also a good time of year to see a charreria.
October- La Romeria. This is the biggest event of the year in Guadalajara when the image of the Virgen of Zapopan returns to the Basilica of Zapopan after visiting many churches during the rainy season. There is a procession from the Cathedral to the Basilica early in the morning of October 12th with hundreds of dancers and many pilgrims. The route goes north on Alcade, then northwest on Camacho to the arch. You can watch at any point along the route that morning. The Virgen should arrive around 10ish or so, give or take an hour. We've also gone to the Cathedral the night before and recommend doing that. In late October you must not miss the mercado for Dia de los Muertos in Morelos Park.
November- Dia de los Muertos. You can eat pan de muertos in Guadalajara and get lots of cempasuchil for your house, but if possible, leave town because the Day of the Dead isn't amazing in Guadalajara although there is still a lot to see. If you can, go to Michoacan, at least to Morelia and to Patzcuaro if possible. I wish we had been able to. If you stay in Guadalajara, there are altars everywhere that are well worth seeing. The Guadalajara Book Fair which is supposed to be the largest book fair in Latin America begins at the end of the month.
December- The fiesta for the Virgen of Guadalupe is on December 13th. We didn't manage to do anything for this either year, mostly because it's also the birthday of one of our children that day, but this is one of the biggest days in Mexico. The closer you get to Mexico City, the bigger it is. And Christmas is always fun. Try eating romeritos and champurrado and bunuelos. There are Christmas markets in Santa Tere and downtown.