No, this is not sign celebrating All Hallow's Eve. The Paris metro entrances were designed in 1900 by architect 

Hector Guimard, who festooned them with cast iron gates and a distinctive Art Nouveau lettering -- which at first may appear to be melting, but are in fact flora-inspired. Photo courtesy of mr_t_in_dc  Search “browse” from The Design of the Paris Metro

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No, this is not sign celebrating All Hallow's Eve. The Paris metro entrances were designed in 1900 by architect

Hector Guimard, who festooned them with cast iron gates and a distinctive Art Nouveau lettering -- which at first may appear to be melting, but are in fact flora-inspired. Photo courtesy of mr_t_in_dc