Why mid-morning wins
By about nine or ten in the morning the dew has burnt off, so the leaf stands up and cuts cleanly. The grass has also had a few hours of light to get going, so it is in good shape to handle being cut, and it has the whole day ahead to recover before night.
That combination — dry leaf, active plant, plenty of daylight left — is what makes mid-morning the standard answer.
Why not first thing
Dew-wet grass has all the same problems as grass wet from rain. It lies down instead of standing up to the blade, it clumps, and it packs into the mower deck. Wet clippings sitting on the lawn on a humid Central Coast morning are also a good way to move fungal disease around.
Why not the middle of a hot day
On a 33-degree January afternoon the lawn is already under heat stress. Cutting it then removes the leaf that is shading the soil, right at the point of the day when the soil least needs to be exposed. The result is browning off in the spots that were already struggling.
It is also the worst time to be behind a mower. Heat and a hot engine is how people end up in trouble.
Late afternoon, with one caveat
Late afternoon works well, as long as the lawn has a couple of hours to dry and settle before dark. Mowing at dusk leaves cut leaf sitting damp through a warm, still night, which is exactly the condition fungal disease wants.
The legal hours in NSW
Under the Protection of the Environment Operations (Noise Control) Regulation, power tools including mowers and blowers must not be used before 7am or after 8pm Monday to Saturday, and not before 8am or after 8pm on Sundays and public holidays.
Worth knowing if you are the neighbour with the early start, and worth knowing that anyone quoting you a 6.30am job is not doing it by the book.