What six months of the year does Spain and France each own the condominium of Pheasant Island?

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Pheasant Island, apparently, is a condominium in the land-property sense of the Word. In all the simple research, it's not possible to discover which six months of the year each country maintains sovereignty.

For example:

"...joint sovereignty of Spain and France, and so administered by Irún (inGuipúzcoa, Spain) and Hendaye (in the Pyrénées-Atlantiques, France), which are in charge of the island during alternating periods of six months."

and:

"...The island is called Isla de los Faisanes in Spanish, Île des Faisans, Île de l'hôpital or Île de la Conférence in French and Konpantzia in Basque."

Given its prominence as a ceremonial site, however, for its smooth operation, the joint sovereignty should be mostly invisible to visitors.