Levels: 100
Reshiram 100% During the following months: February, April, June, August, October, and December.
Zekrom 100% During the following months: January, March, May, July, September, November.
Note: After obtaining all 8 badges, and beating the Pokemon League, you can investigate the mysterious Fogbound Lake and find Reshiram and Zekrom locked in battle every 2nd month.