It is not the day when the US defaults, at least not defaults on its debt. It will begin "defaulting" on various financial obligations, such as not paying due bills on time and in full, but since this is something Europe's periphery has been doing for years, it is hardly catastrophic.

As a reminder, the technical definition of default is being unable to pay the interest or maturity on one's debt - for the US this is not an issue (at least not for the near term), as the revenues brought in by taxes more than cover cash interest requirements. It is, however, the day past which the US government will slowly but surely have to begin shutting down non-vital services and cut spending for non-core services, as it is forced to begin prioritizing who gets what from the remaining money. In short: it the day when America has no choice but to live within its means (no wonder the crank spending to infinity "Magic Money Tree" brigade - the same that has no idea that IRR on government "investments" is always below zero, is screaming bloody murder).