Last semester I ran a THEORY DAY AT UMCP. Below I have ADVICE for people running theory days. Some I did, some I didn't do but wish I did, and some are just questions you need to ask yourself.
1) Picking the day- I had two external speakers (Avrim Blum and Sanjeev Arrora) so I was able to ask THEM what day was good for THEM. Another way is to pick the DAY first and then asking for speakers.
2) Number of external speakers- We had two, and the rest were internal. The external speakers had hour-long talks, the internal had 20-minute talks. This worked well; however, one can have more or even all external speakers.
3) Whoever is paying for it should be told of it towards the beginning of the process.
4) Lunch- catered or out? I recommend catered if you can afford it since good time for people to all talk. See next point.
5) If its catered you need a head count so you need people to register. The number you get may be off- you may want to ask when they register if they want lunch. Then add ten percent.
6) Tell the guest speakers what time is good for them to arrive before they make plans so you can coordinate their dinner the previous night.
7) If have the money and energy do name tags ahead of time. If not then just have some sticky tags and a magic marker.
8) Guest speakers- getting them FROM Amtrak or Airport to dinner/hotel --- give them a personal lift (they may be new to the city and a bit lost). Getting them from the event back TO the airport or amtrak, can call airline limo and taxi. (though if can give a ride, that's of course good.)
9) Pick a day early and stick with it. NO day is perfect, so if someone can't make it, or there is a faculty meeting that day, then don't worry about it.
10) Have website, speakers, all set at least a month ahead of time. Advertise on theory-local email lists, blogs you have access to, and analogs of theory-local for other places (I did NY, NJ, PA). Also email people to spread the word.
11) Advertise to ugrads. Students are the future!
12) If you are the organizer you might not want to give a talk since you'll be too busy doing other things.
13) Well established regular theory days (e.g., NY theory day) can ignore some of the above as they already have things running pretty well.