Big Friday is a term used in athletic populations (predominately strength athlethes) to describe the final training session subsequent to a weekend of recuperation. Big Friday is usually (but not exclusively) a heavy or tough training session where the goal is to achieve a big training performance, however it is also associated with strong intent and focus towards the final training session regardless of what weight is prescribed in the training session.
Bob: “I cannot wait for Big Friday, What do you have for big friday?”

John: “I got heavy squats and pulls mate, It is going to be a good one
by thebingerer March 9, 2018
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It's Casual Friday to the next level, where people come to work without fixing their bed hair.
Joe: "Whoa, what's with your hair, dude? Rough night, last night?"

Bob: "Didn't you get the memo? It's Big Hair Friday!"

Joe: "Gee, I can't believe I forgot it's BHF!"
by GeekSquadRejects March 11, 2010
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