These docs are for an old release.
Info on upgrading to v4
slotLabelInterval
The frequency that the time slots should be labelled with text.
If not specified, a reasonable value will be automatically computed based on slotDuration. When specifying this option, give a Duration-ish input, like "01:00" or {hours:1}. This will cause the header labels to appear on the hour marks, even if slotDuration was hypothetically 15 or 30 minutes long.