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You can check for interview availability (without needing to be logged in) at TTP - Schedule Appointment

This page includes a checkbox you can use to show locations with the soonest availability, although they tend to be few in number and near border towns.

You may also wish to consider:

• Enrollment on Arrival, which can be done while clearing immigration during international return to the US: Global Entry Enrollment on Arrival
• Some locations offer walk-in interviews even if you do not have an appointment: Global Entry Walk-In Interviews
• If you will be passing through MIA, their GE office nearly always has immediate availability and is open very late.

Note also that a location that has literally zero availability for future interviews today, may pop up with available slots tomorrow (or even later today), presumably as people with scheduled appointments do EOA or walk-in thus freeing up their original time slot. So it may be worth checking back periodically.