Rollover is the procedure of moving open positions from one trading day to another.
Most brokers and trading platforms perform the rollover automatically at the end of each trading day, usually at 24:00:00 or midnight server time. This is when the markets reset for the next trading day and swap fees/carry over costs are added to any open positions.
Most people are aware of this period, but there is a lesser-known rollover period that occurs for the New Zealand Dollar 2 hours before the main rollover each trading day at 22:00:00 (or 10pm server time) and this will affect any trading pair that features the NZD such as EURNZD, NZDUSD or even NZDCAD.
This second rollover period has very similar characteristics to the main market rollover whereby the spreads will usually spike to very high levels (compared to the daily average) and can affect any open positions.
I have included some images to show the effect on the spread average around this time for your understanding:
Note below the direction of the spread spike, depending on which currency pair is the base currency (first in the pair) or which is the quote currency (second in the pair).
In conclusion, the forex markets can be quite complex and difficult to trade without understanding the full spectrum of risks, but once you are able to learn more about these types of events alongside the importance of managing risk and open positions, it becomes possible to succeed as a trader.