A shared calendar, like a PTO calendar, works best when the daily/weekly events are pushed to a Slack channel. This is how I do it for free, securely, and without needing to pay for a Slack bot/app.

1. Add the calendar to your Google Calendar, usually as a webcal link.
2. In Slack, connect your Google Calendar.
3. In your team’s Slack channel, as Google Calendar as an App/Integration.
4. Select the shared calendar and pick daily/weekly updates.