Jupiter-Saturn conjunctions are a phenomenon which re-occurs at time intervals of a little while less than 20 years, is visible from Earth by the naked eye and can happen in two basically different ways according to the number of planetary alignments: the single or unique J-S conjunction and the multiple (that is triple or double) J-S conjunction. This last way of being of the J-S conjunctions depends on a particular position of the Earth on its orbit at the moment of the heliocentric J-S conjunction (see 
'Liverpool posters' among the labels, 'etichette') and is therefore (usually) a seldom one: it can be missing also for centuries. For instance, the last occurrence was in 1981 (see diagrams here above), the next one will be in
2238-2239.
It has been demonstrated by the blog's author that this phenomenon, which lasts seven/eight months (almost like a pregnancy) in the night sky, was carefully observed already in the antiquity and that it was allegorized in the image of the phoenix (the heliopolitan bennu).
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