The following two syntaxes work perfectly:
=IIf([End Date] Is Null,Format$([Start Date],"mmmm dd"", ""yyyy"),Format$([Start Date],"mmmm dd"", ""yyyy") & " to " & Format$([End Date],"mmmm dd"", ""yyyy"))
=Trim("Thank you for agreeing to be a faculty member for the " & [Activity Title] & "...