Calendar Effects (Day-of-Week & Turn-of-Month) API
The two best-documented calendar anomalies in equities — the day-of-week effect and the turn-of-month effect — measured live across a cross-asset universe from Yahoo Finance daily history, no key, nothing stored. Decades of research show returns are not spread evenly through the week or the month: the turn-of-month effect — the cluster of the last trading day of a month and the first few of the next — has historically captured the bulk of the entire month's gain while the rest of the month drifts; and the day-of-week effect (the old "Monday effect" and its kin) shows some weekdays running persistently stronger than others. This API quantifies both directly. The turnofmonth endpoint splits an instrument's history into the turn-of-month window (the last trading day plus the first three of each month) versus the rest, and returns the average daily return and win-rate of each, the spread between them, and the share of the total return earned inside that handful of days. The dayofweek endpoint returns, for each weekday, the average daily return, win-rate and sample size, with the best and worst day. The screener endpoint ranks the cross-asset universe by the strength of the turn-of-month effect, so you can see where the calendar edge is biggest. This is the day-of-week / turn-of-month calendar-anomaly cut — distinct from the month-of-year seasonality APIs (equity-index, FX, commodity) and the crypto-only intraday/day-of-week seasonality API. Patterns are descriptive, not predictive.
api.oanor.com/calendareffects-api
