See my other post below for details on previous years. But check this out.
2011: 22,312 (Dayton Hamvention direct query – WA8APB) Sunny and warm all 3 days, upper 70′s to lower 80′s. Read the 2011 recap.
2012: 24,483 (DARA Meeting) The weather for this year was very pleasant with temperatures in the 80′s and sunny all 3 days. Read the 2012 recap.