What does the ‡ symbol in the Chart Supplement (A/FD) designate?
The ‡ symbol in the A/FD specifies that the location observes daylight savings time.

This is from the A/FD legend, which can be found online by going to the online digital A/FD and searching for an airport.  The A/FD Legend link will then be at the top of the search results (unfortunately the link changes every cycle so I can't include an exact link).


  6 TIME CONVERSION   
  
  Hours of operation of all facilities are expressed
  in Coordinated Universal Time (UTC) and shown as ‘‘Z’’ time. The
  directory indicates the number of hours to be subtracted from UTC to
  obtain local standard time and local daylight saving time UTC–5(–4DT).
  The symbol ‡ indicates that during periods of Daylight Saving Time
  effective hours will be one hour earlier than shown. In those areas
  where daylight saving time is not observed the (–4DT) and ‡ will not
  be shown. Daylight saving time is in effect from 0200 local time the
  second Sunday in March to 0200 local time the first Sunday in
  November. Canada and all U.S. Conterminous States observe daylight
  saving time except Arizona and Puerto Rico, and the Virgin Islands. If
  the state observes daylight saving time and the operating times are
  other than daylight saving times, the operating hours will include the
  dates, times and no ‡ symbol will be shown, i.e., April 15–Aug 31
  0630–1700Z, Sep 1–Apr 14 0600–1700Z.