If you were expecting to receive a check-in reminder and it never came, it's likely because a team administrator marked the day as a team holiday, or you marked yourself as on vacation.
If you are an Administrator or Team Manager, you can check to see if a day is marked as a holiday in Team Settings - Holidays.
To check if you marked yourself on vacation, go to Settings - Vacation / Leave. If you see a blue outline on the date, then you are marked as out. Just click the date to change it.
If you didn't receive your reminder because of one of those reasons, you can manually checkin by clicking Check in in the top navigation bar or typing "/checkin now" in Slack.