If you were expecting to receive a check-in reminder and it never came, it's likely because an administrator marked the day as a holiday or you marked yourself as on vacation.

If you are an Administrator, you can check to see if a day is marked as an account holiday in Account Settings - Holidays.
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To check if you marked yourself on vacation, go to Settings - Planned absences. If you see a blue outline on the date, then you are marked as out. Just click the date to change it.

If for some other reason you didn't receive your reminder, you can manually check in by clicking Check in in the top navigation bar or typing /checkin now in Slack.