If your car's air-conditioning system blows only warm air on hot days, it's probably because it is low on refrigerant, and the most likely cause is a leak somewhere in the system.
Though many vehicle owners think they need to recharge or "top off" their air-conditioning system with refrigerant on a regular basis, such as annually, that's not the case. If the air conditioning stops working (but the fan continues to blow warm air), a leak in a hose, connector or other part of the system is probably the culprit. During the winter, you may notice that the defroster isn't clearing fogged up windows, and that also is a sign you have a leak. Most defrosters engage the air conditioning to dehumidify the air.
If there is a leak, the air-conditioning system on most modern vehicles is designed to shut down once the refrigerant level drops low enough in order to prevent damage to the air-conditioning compressor. Note that we are talking about "refrigerant," not Freon, the brand name for a type of refrigerant that is no longer manufactured in the U.S. because it can damage the ozone layer. Contemporary vehicles use a refrigerant known as R-134.