How the end of ranked season actually works?

Okay I was wondering because one of my friends last season went to plat before the update and yet he was gold on the boards, so I was wondering there's spesipic date to the end of the season and you can still climb up rank after the season ends but before it updates? I really don't get it, anyone please help me uderstand how riot works? Do I finish plat now, and if I fall to gold, does it gonna End up me getting gold frame?