In The Technique of Rectification, Charles Jayne addresses the question of the timing of transits. He writes,
The effect of a Saturn transit never preceeds the perfection of the aspect, but always follows it.
The effect of the Sun also lags behind the exact aspect, but not by as much.
Effects of secondary (aka Major) progressed Mars often preceed the exact aspect, sometimes considerably (Jayne recommends orbs as tight as 05’ to 15’!)
Neptune is the planet whose influence most consistently leads the time of the exact aspect.
He writes, Uranus is the most consistently exact, its influence matching the time of the exact aspect to within 02’and usually near 00’. This is especially useful in connection with the Angles of the chart.
The effects of the secondary (aka Major) Moon are also very accurate.
Paraphrased from The Best of Charles Jayne (book four, The Technique of Rectification), page 143.
In connection with the effects of transits when considering the timing of events, Elbert Benjamine (writing as C. C. Zain) wrote
“The events which many people believe to be occasioned by transiting planets are really coincident with major-progressed aspects which have formed heavier temporary stellar aerials. Under such circumstances the transit, which merely aids the Major Progression, and which fails to work when no such Major-Progressed aspect is present, is often accredited full responsibility for attracting the event.”
Natal Astrology, Part 2; Progressing the Horoscope.
Many other astrological writers concur with Benjamine on this point.