Let’s say a PC is fighting against a monster. The monster got the highest initiative so it goes first. After the fight’s gone on for a little bit, the monster runs back, out of the range of the PC’s run range. The player, instead of chasing, readies an action to attack the monster when it charges at him. The monster then charges at him.
Now, does the PC get to hit the charging monster before the monster does its melee basic attack or bull rush, or does it occur after? Also, does the player’s next turn occur after the monster’s current turn ends?