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>> “29 Elul is Sunday, Sept. 13th.” <<
To be specific, Sundown, Sept 13 begins 29 Elul.
God doesn’t go by man’s days.
I believe sundown on our Sunday, Sept. 13th is actually the start of Tishri 1.
The days go from sundown to sundown. Elul 29 starts at sundown on Saturday, Sept. 12 and ends at sundown Sunday Sept. 13.