Mars and Saturn in conjunction: the tradition's least ambiguous aspect
The tradition qualifies almost everything. Aspects are read through dignity, reception, sect and house, and a configuration that looks severe in isolation may be harmless in place. There are a few exceptions, and the conjunction of the two malefics is the clearest of them.
Why this pair is treated as it is
Mars and Saturn are malefic for opposite reasons, and that is the point. In the humoral scheme Mars is hot and dry in excess, Saturn cold and dry in excess. What they share is dryness — and dryness doubled, in this system, is what removes the capacity to adapt.
Their combined significations run consistently across the authorities: destruction that is also slow; violence with a long aftermath; structures that fail under force; scarcity produced by conflict. Not calamity in general, but a particular kind — the sudden thing that leaves a lasting deficit.
What modifies it
Three things, in order of weight.
Sect. This matters more than anything else and is the most frequently omitted. Saturn belongs to the day, Mars to the night. In a diurnal chart Saturn is in sect and behaves more moderately while Mars is out of sect and behaves worse; by night the reverse. A malefic in sect is still a malefic; it is a malefic acting within limits.
The sign. A conjunction in a sign ruled by one of the two is a different matter from one in the domicile of a benefic. In Virgo, for instance, Mercury disposits the pair, and a mercurial disposition tends to turn the significations towards disputes, documents and words rather than towards physical destruction. Earth signs generally slow the manifestation; the sign's ruler tells you in what department.
Reception and witness. A benefic casting a ray into the conjunction does not cancel it but changes its outcome — the classic reading is damage that is survived, or loss that is later recovered. Reception between the malefics and the lord of the relevant house matters just as much.
The mundane reading
Mars–Saturn conjunctions occur every two years or so, which places them below the great Jupiter–Saturn conjunctions in the mundane hierarchy but well above ordinary transits. They are read in the ingress chart for the year and through the angles of the place.
The traditional warning is against elections: beginning anything on the exact day of the conjunction is held to load the matter with both malefics from the outset, and no favourable detail elsewhere in the chart is considered to compensate. This is one of the few pieces of electional advice on which the authorities do not disagree.
The natal reading, and a caution
A natal Mars–Saturn conjunction is a real testimony and a badly overread one. What it describes is a constitution in which effort meets resistance, frequently including the native's own — a hardness that can be endurance or obstruction depending on the rest of the chart.
It is not a sentence. Whether it does harm depends on which houses the two rule, whether they are in sect, whether they receive each other, and what else witnesses them. A conjunction in the eleventh, in sect, with a benefic in aspect, in the chart of someone whose work requires persistence, is close to an asset.
The discipline is the one that applies to the whole system: a configuration is a testimony, and a testimony is not a judgement until it has been weighed against the others.
Sect, and the difference it makes to both malefics, is taught from the first year of the course.