Working calendars, holidays and planned gaps
Count durations, lags and delays in working days, add holidays, and decide what a planned gap between linked tasks means.
Working days and holidays
Project settings, Scheduling: choose the days of the week that are working days and add holidays. Durations, lags, delays and float are then counted in working days, and no forecast lands on a day nobody works.
With no calendar every day is a working day, which is how every project counted before calendars existed. A task with no delay and nothing pushing it keeps its planned dates on any calendar.
Planned gaps
When the plan leaves a gap between two linked tasks, the project chooses what it means. Keep the gaps you planned (the default): a slip moves the next task by the same amount. Use planned gaps as slack: a slip uses up the gap before the next task moves.
Before you save, the page shows which tasks would move under the new settings.