When Does Winter Start in Saudi Arabia — and Which Months Count as Winter?

The short answer

There are two defensible answers. Meteorological winter is the fixed three-month block of December, January, and February, the grouping used for climate statistics; astronomical winter starts at the December solstice and ends at the March equinox, as NOAA explains. In everyday Saudi usage the cool season is talked about more loosely, stretching from around November into March depending on the region.

Two definitions, two start dates

Meteorological seasons divide the year into four fixed three-month blocks aligned to the annual temperature cycle. Winter is December through February — tidy, comparable year to year, and the basis of most published averages.
Astronomical seasons are set by the solstices and equinoxes, so winter begins around December 21 and the date wobbles by a day or so each year.
Climate normals — the 30-year averages compiled through programs such as NOAA's WMO Climate Normals — use the fixed monthly framing, which is why month-by-month tables are easier to compare than season labels.

Summary: for planning, think in months; for astronomy, think in solstices.

Why "winter" lands differently across the country

Saudi Arabia is large enough that one national winter date is misleading. The broad pattern:

Interior plateau (Riyadh and the central region) — cools off fastest and furthest, with the sharpest day-to-night swing. Nights in the core winter months can be genuinely cold while afternoons stay mild.
Red Sea coast (Jeddah, and the Makkah area inland of it) — the mildest version of winter, moderated by the sea, with humidity more noticeable than cold.
Southwestern highlands (the Asir region and Taif) — the coolest part of the country for its latitude, because of elevation rather than season alone.
Gulf coast (the Eastern Province) — cool and often damp or windy rather than cold.

The National Center for Meteorology is the official Saudi source for forecasts, alerts, and regional climate information, and Visit Saudi is the tourism authority's reference for how the regions differ. For the two-city case specifically, see Riyadh versus Jeddah on climate.

Summary: elevation and coastline shape Saudi winter at least as much as the calendar does.

Rain follows a different schedule

Winter is not uniformly the wet season across the country:

Across much of the interior and north, what rain falls is concentrated in the cooler half of the year, often as scattered, short-lived events rather than steady wet spells.
The southwest highlands run on a different rhythm, with their wetter period skewed toward spring and summer.
Totals are low almost everywhere, so a monthly average can be dominated by a single event — which is exactly why 30-year normals are more informative than any one year.

Summary: "rainy season" means different months depending on which part of the country you mean.

Why Hijri months are no guide to the season

A common mix-up: assuming a named Hijri month implies a time of year. It does not. The Hijri calendar is purely lunar and drifts about 11 days earlier against the solar year annually, so Ramadan or Dhu al-Hijjah can sit in the cool season in one decade and the hot season in another — the mechanics are in how the Hijri calendar works. If you are matching a trip to the weather, work in Gregorian months and convert the Hijri dates afterward.

Putting it together for planning

Use December–February as the core of winter, and treat November and March as shoulder months that often feel part of the cool season.
Expect wide day-to-night ranges inland and much smaller ones on the Red Sea coast.
Check regional forecasts rather than national ones, since conditions in the highlands and on the coast diverge sharply.
Compare Gregorian months against which months have the mildest weather and, for pilgrimage timing, Makkah's weather by season.

Summary: winter formally starts on December 1 by the meteorological definition and around December 21 by the astronomical one — but the cool season people actually notice runs wider, and varies by region.