A regular clean keeps a home under control. A grand ménage resets it. The difference is everything a weekly visit reasonably skips: inside the appliances, behind the furniture, and every surface above eye level.
In the kitchen, that means the inside of the oven and fridge, a degreased range hood and backsplash, and cupboard doors wiped inside and out. In bathrooms, it's descaled glass and taps, grout scrubbed, and the base of the toilet cleaned properly.
Around the house, baseboards and door frames are washed by hand, light fixtures and vents are dusted, and windowsills and tracks are cleared of a season's grit. These are the spots that make a home look tired even right after a quick clean.
Spring is the traditional time, after months of closed windows and winter salt tracked indoors. But fall works just as well before the holidays, and a deep clean is always the right first visit before starting a regular schedule.