The AIRAC cycle was adopted in 1964 and further improved over the years. Key are the worldwide effective dates with 28 days intervals (e.g. 4 August 2005, 1 September 2005, 29 September 2005, …). Effective days are always on a Thursday. It may look indeed like a long period, 28 days, or even 56, but it should be understood that aeronautical information changes, mostly published through so called AIRAC Amendments.
Back to our Navigraph Cycle 2004, effective from March 26th. By following this link you reach the download section of Navigraph.