Although normally a very faint object, Holmes became notable during its October 2007 return when it temporarily brightened by a factor of a million, in what was the largest known outburst by a comet, and became visible to the naked eye.

[5] It also briefly became the largest object in the Solar System, as its coma (the thin dissipating dust ball around the comet) expanded to a diameter greater than that of the Sun (although its mass remained minuscule).

Aided by the computer predictions of Brian G. Marsden, the comet has been observed on every subsequent return.During its 2007 return, Holmes unexpectedly brightened from a magnitude of about 17 to about 2.8 in a period of only 42 hours, making it visible to the naked eye.

A. Henríquez Santana on Tenerife in the Canary Islands; minutes later, Ramón Naves in Barcelona noticed the comet at magnitude 7.3.

[13] Although large telescopes had already shown fine-scale cometary details, naked-eye observers saw Holmes as merely star-like until October 26.

In November 2007, the coma had dispersed to a volume larger than the Sun, briefly giving it the largest extended atmosphere in the Solar System.

[20] However, researchers at the Max Planck Institute suggest in a paper published in Astronomy and Astrophysics that the brightening can be explained by a thick, air-tight dust cover and the effects of H2O sublimation, with the comet's porous structure providing more surface area for sublimation, up to one order of magnitude greater.