Never heard of this expression before so it is most likely an image. The translation for "jour d'Église", is "Church day". But this you probably know already.
It could be suggesting something related to sundays, the seventh day in the Bible...
But given the context in the sentence, it could be related to "l'effet donné à la lumière lorsque projetée au travers des vitraux d'Église" = the effect given to the lights when projected through stained glass in Churches, which is a shadowy or a clair-obscur effect...
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AQ = 39; EQ = 14; IQ = 137; Eye Expression Test = 23
Diagnosed in 2014
Overload of social interactions numb the deepest thoughts.