While a Holter monitor continuously records your hearts rhythms over a period of one to two days, an event monitor is used to record rhythms only when you feel a symptom of irregularity coming on. Event monitors are small devices that are worn over a longer period (weeks to months, typically one month) than Holter monitors. Event monitors are worn as much as possible every day to increase the chances of recording your rhythm when you experience symptoms.
The monitor is always on but will only store your rhythm when you or your caregiver pushes the button on the monitor. Most monitors will save the rhythm for several seconds of rhythm before the button is even pushed. The rhythm is also saved for a period after the button is pushed.