According to Buzzfeed it's 7 PM on a Monday night. Unfortunately, that is a vague answer that doesn't reveal the most helpful information. If you want to know the best time to post you need to know the specific targeted subreddit. Each subreddit has a unique posting signature that varies. For example, r/news has a vastly different ideal posting time than r/shutupandtakemymoney.
The best time to post in r/news is Thursday at 8 PM EST
The best time to post in r/shutupandtakemymoney is Tuesday at 12 PM EST
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