Probably the first half of the 1920s.
There is an enormous amount of information embedded in this pattern. Let's start with terminology. I wouldn't call these knickers, I'd call them breeches.
By way of contrast, here is a nice pair of ladies' knickers illustrated on page 61 of the Charles Williams Stores catalog for Spring/Summer, 1926. This illustration appears in the ladies clothing section of the catalog dedicated to rugged, unfussy skirts and suits classified as "sport apparel." It's one of two models of knickers offered.
Charles Williams Stores also carried ladies breeches, but you need to turn to page 284, in the men's section of the catalog, to find them:
This is the only model offered for ladies' breeches in this particular catalog. By contrast, there were four different models offered for men. Historically in the world of bespoke clothing, ladies' riding apparel was generally made by tailors (who were men) rather than dressmakers (who were women.) It may still be for all I know; that's not a world to which I have much visibility.
In 1926 Charles Williams didn't seem to offer any other trouser-type garments for ladies. Knickers and gym bloomers were offered for girls.
The most obvious difference between the two garments is the enormous amount of ease through the seat of the breeches, necessary if one intends to wear them for riding, but really very practical as well for skating, strenuous hiking, etc. Taking a rough measurement of the pattern pieces, I estimate that this pattern with a stated hip measure of 38 inches will be about 54 inches through the hips. Note that modern riding breeches aren't anywhere near as full-seated, so even in utilitarian garments, styles change.
Returning to the Butterick pattern, the description states that these "knickers" were
Suitable for General Sports Wear, Riding, Motoring, Hiking, etc.
This is not a trivial garment to make at home. Here are the instruction sheets.
On top of the fairly complicated construction, the idea of making these in a napped fabric makes me a little light-headed. Also, by my count, at least 22 (hand-worked) button holes are required. It could be more. The illustration seems to show that in view B, the breeches legs are laced up the back; the instructions are entirely silent on the details of this view.
Further research will be required to determine if Butterick offered patterns for both breeches and knickers, or if this was their only offering of trousers for women. Fashionable slacks for women won't show up until the 1930's. It would be fascinating to know how many women simply opted for wearing men's trousers for sport or work wear.