Tier changes happen on the first day of each month. All newly introduced fully-evolved Pokemon start in the OU tier, then they drop to the lower tiers as time passes. Non-evolved Pokemon start at NU and can work their way up through bans or through usage at a 4.52% rise/drop cutoff. It's easiest to explain the rise/drop cutoff with an example. If a UU Pokemon isn't used on at a minimum of 4.52% of all teams in a month, it would drop down to RU. Or, if that Pokemon was used on at least 4.52% of OU teams for that month, it would rise to OU. I know you didn't ask for all of that, but hopefully this makes it easier to understand why tier changes happen at the beginning of each month: since the tier changes themselves are based around a month of collecting usage statistics.
