Riders and spectators alike are often drawn to the beauty and uniqueness of the riding costumes of old. Historically speaking, most women who rode did so for pleasure or sport, and could afford special riding clothes. (Not unlike today's riders!) They would never think of riding in their evening gowns or city clothes. The practice of riding aside in everyday clothes was largely limited to tourists and the American West, where everything - including horses and clothing - was expected to fill many functions.
As with any fashion, riding habits changed over the years, for the seasons, and with the geography. The same woman who wore a heavy wool habit in England's winter might have a linen habit in India. When reproducing historical habits, you should research your chosen time to learn about the fabrics, cuts, and construction techniques used. Remember, too, that your habit will not look quite right without the proper undergarments, which might include stays or corset, petticoats, shirts, pantalettes, and so on. In the process, you will doubtless also discover what hat, gloves, whip, and other accessories are appropriate. Faithfulness to the original is of course a personal decision, and varies from a handmade stitch-to-stitch perfect reproduction using custom-woven fabric to quick polyester costumes intended for easy washing after a parade.
Ideally, your outfit would be from the same time period as your saddle. This only a ï¿½perfect worldï¿½ scenario, though, as most useable side saddles are post-1900, and that would keep us from using too many lovely habits! In the very likely case that your saddle does not match the era of your habit, you should know and be able to explain the differences.
Likewise, if you adapt the habit for safety, budget, or time issues, you should be able to explain why, and what the original would have been like. For example, you might incorporate a safety helmet instead of a straw boater, or use a poly blend instead of $100/yd wool. You might choose to use Velcro instead of buttons, or leave the seam closest to the pommels unstitched to help prevent the skirt from being caught in case of a fall. Perhaps a zipper instead of tiny buttons to facilitate a quick change. It's OK to change history - so long as you don't pass it off as "how it really was."