OK, I can give you a rough idea of when each build-off happens, dates may vary by a day or two here and there... As each build-off becomes current it usually gets its own section at the top of the forum list and when it is finished it moves to the archives:
http://www.ratrodbikes.com/forum/index.php?forums/the-archives.40/
The main Build-off which is the biggest and best usually runs from May 1st to the first week of August some time.
Your best bet for information regarding rules and the like can be found in the archives of past competitions:
http://www.ratrodbikes.com/forum/index.php?categories/rrb-build-off-competitions.100/
There has lately been two smaller additional build-offs per year, one at the start of the year usually Jan 1st to April 1st normally called the "Winter Build-Off" but this year has the theme "Off-Road" so basically bikes that are meant to be ridden off-road: MTB, BMX, Klunker, Hybrid etc...
Information on that is currently here:
http://www.ratrodbikes.com/forum/in...ff-orbo-bmx-mtb-bomber-klunker-jan-apr.88672/
The other smaller build-off is the one running currently until Jan 1st, that is the 4th annual Muscle Bike Build-Off (MBBO#04 for short) this build-off has gathered quite a following the last couple of years, so there is a fair chance it will continue running in the same timeslot for the future years too.
Information on this build-off can be found at the top of the current forum list:
http://www.ratrodbikes.com/forum/index.php#muscle-bike-build-off-2015.116
Let me know if you have any other questions and I will do my best to answer them, usually a good search through the archives will answer most questions though!
Luke.