The Rain Calendar

The durable rule first, because it doesn't move with the weather: the danger is worst when rain arrives on a road that has been dry, and it falls as the rain continues. That holds in any month and any country. What a calendar adds is the ability to anticipate it.

Current context (worth updating as patterns shift): Kenya's rainfall is broadly seasonal — long rains around March to May and short rains around October to December, with dry spells between. Two consequences follow for drivers.
First, the start of each rainy season is the highest-risk window on the calendar: the dry spell before it has been long, so the accumulated film is thickest, and the first storms fall on the dirtiest roads of the year.
Second, the first rain of any dry spell carries the same effect in miniature — a shower after a dry fortnight in the middle of a wet season is still a first rain.
Rainfall patterns are shifting, and regions differ considerably. Treat the months as a guide to when to expect the effect, never as a licence to relax outside them.

The driver's habit is small and pays for itself: when you notice the first drops after a dry spell, change how you're driving before you feel the road change — because by the time you feel it, you're testing it.