As I understand it, technically, it's 20 minor league days. The reasoning behind the 20 days has to do with how a player's options are calculated. You need to keep a player down for 20 days for him to burn an option. If he is down for less than 20 days, then all of that time in the minors is actually counted as ML service time and he keeps his option. So by keeping him in the minors for 20 days, you're burning an option year for him, but you're gaining an extra year before arbitration.