On which day did ARPANET suffer a 4 hour long network crash?

The story behind the answer

October 27, 1980 is correct because that day ARPANET—the precursor to the modern Internet—suffered a four-hour collapse when a software bug in the network’s Interface Message Processors triggered a storm of routing updates. The flood overwhelmed the network, effectively shutting it down until engineers patched the problem.

This failure became a landmark lesson in designing robust routing and congestion-control mechanisms. It helped inform the stability features used in later Internet protocols. A memory tip: two famous ARPANET October dates—Oct 29, 1969 (the first “LO” message) marks its birth, while Oct 27, 1980 marks its first major meltdown—ten-plus years later, in the “’80 crash,” reminding us that early networks had to break before the Internet could learn to bend without failing.