Checking the calendar before the session is basic hygiene. High-impact events (cpi, nfp, fomc, pmi, GDP) produce spikes, slippage, and widened spreads, and many prop-firms have a news-trading-rule around them.
Most calendars are free and show previous, forecast, and actual values.
Example: the calendar shows CPI at 8:30 a.m. and an FOMC decision at 2:00 p.m. on the same day. A day trader plans to be flat at both times and to expect a wide range.
Related: cpi, nfp, fomc, news-trading-rule, pmi