If no bids file by April 15, site selection will proceed with nothing but write-in bids. The deadline for filing bidding papers to be eligible to win the bid as a write-in is the close of site selection balloting at Westercon 70 in Tempe this July. If no bids file bidding papers, or if no filed bid wins the voting, or if None of the Above wins, then the Westercon Business Meeting gets to decide where to hold Westercon two years hence. Again.
Note that while the Westercon Bylaws recommends that Westercon be held over the American Independence Day weekend, this is not required. It is merely a strong suggestion.