♃ is an astrological and astronomical symbol representing the planet Jupiter.
In Romance languages, the word for Thursday originates from Latin Iovis dies, i.e. “day of Jove” (dijous, giovedì, jeudi, joi, jueves, xoves). In Germanic laguages, this day is connected with god Thor or Donar (donderdag, Donnerstag, Thursday, torsdag).
According to Wikipedia, the “less-than-intuitive symbol” ♃ has been variously interpreted as the letter ζ (for Ζεύς, “Zeus”), lightning bolt, or even “Egyptian hieroglyph for the eagle”. For me it is easier to remember as a symbol for Thursday, the fourth day of the week. I write the number “four” like 4, which also happen to be similar to Ч, the first letter of the Russian word четверг (Thursday).
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