According to DIRECTIVE 2006/7/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 15 February 2006 concerning the management of bathing water quality:

Bathing water quality assessments shall be carried out:
  1. in relation to each bathing water;
  2. after the end of each bathing season;
  3. on the basis of the set of bathing water quality data compiled in relation to that bathing
  4. in accordance with the procedure set out in Annex II.
Prior to the implementation of DIRECTIVE 2006/7/EC a different, now obsolete evaluation scheme was used.