RubyConf Australia has always been held within the first two weeks of February, and this timeframe generally has worked quite well.
Potential events that may conflict:
Of course, clashing with any of these events is not a serious problem (with the exception of Rails Camp New Zealand, as that is geographically close and many of their attendees also want to be at RubyConf Australia). The bigger constraint in the early stages of planning is confirming a venue that is available in February.