Calendar and holiday regressors

Calendar effects are part of the model specification. In seasight, they are therefore treated as explicit candidate choices rather than hidden package data. This keeps the package focused on model search, diagnostics and reporting while leaving production calendars under the control of the user or institution.

The practical rule is:

• use X-13 or seasonal built-ins for standard effects where possible;
• use build_user_xreg() for institution-supplied moving-holiday dates;
• pass the resulting named regressors through td_candidates;
• inspect with_td, td_name and the report’s “TD regressor” column.

seasight intentionally does not ship broad holiday datasets such as holiday or chcal. National holidays, bridge days, school calendars, religious holidays and trading calendars are source- and sector-specific. A generic bundled calendar can easily be wrong for a production series.

Built-in Easter handling

For Easter, the simplest path is usually to let X-13 handle the effect through the model specification. seasight exposes this through include_easter.

library(seasight)
library(seasonal)

y <- AirPassengers

res_easter <- auto_seasonal_analysis(
  y = y,
  include_easter = "auto",
  engine = "auto"
)

res_easter$table[1, c(
  "model_label", "with_easter", "easter_window", "with_td", "td_name",
  "score_100"
)]

In "auto" mode, X-13 tests whether Easter improves the model and selects among its supported Easter windows. The result columns report the effect actually retained, not merely the requested mode. Use include_easter = "always" only when the effect is part of the production policy for that series; this fixes easter[easter_len]. Use include_easter = "off" when Easter is known to be irrelevant or is handled elsewhere; this disables both inclusion and testing.

Precomputed trading-day regressors

For working-day or trading-day effects, many institutions already maintain calendar regressors. Pass those directly as a named list. The names are carried through into the candidate table and HTML report.

dates <- seq(as.Date("1949-01-01"), by = "month", length.out = length(y))

days_in_month <- function(date) {
  first <- as.Date(format(date, "%Y-%m-01"))
  next_month <- seq(first, by = "month", length.out = 2)[2]
  as.integer(next_month - first)
}

length_of_month <- vapply(dates, days_in_month, integer(1))
td_len <- ts(length_of_month - mean(length_of_month), start = start(y), frequency = frequency(y))

res_td <- auto_seasonal_analysis(
  y = y,
  td_candidates = list(length_of_month = td_len),
  td_usertype = "td",
  include_easter = "auto",
  engine = "auto"
)

res_td$table[1, c("model_label", "with_td", "td_name", "td_p", "score_100")]

The example above is deliberately simple. In production, the regressor should come from the relevant business, statistical or national-accounts calendar.

Moving holidays with build_user_xreg()

For holidays with known dates, such as Diwali, Lunar New Year, Ramadan-related effects or local events, use build_user_xreg(). Internally this uses seasonal::genhol() and then aligns the resulting regressor to the target series.

diwali_dates <- as.Date(c(
  "2018-11-07", "2019-10-27", "2020-11-14", "2021-11-04",
  "2022-10-24", "2023-11-12", "2024-11-01"
))

cny_dates <- as.Date(c(
  "2018-02-16", "2019-02-05", "2020-01-25", "2021-02-12",
  "2022-02-01", "2023-01-22", "2024-02-10"
))

holiday_xreg <- build_user_xreg(
  y = y,
  holidays = list(
    diwali = list(dates = diwali_dates, start = -7, end = 0),
    lunar_new_year = list(dates = cny_dates, start = -3, end = 3)
  ),
  td_usertype = "holiday"
)

names(holiday_xreg)

res_holiday <- auto_seasonal_analysis(
  y = y,
  td_candidates = holiday_xreg,
  include_easter = "auto",
  engine = "auto"
)

res_holiday$table[1, c(
  "model_label", "with_td", "td_name", "with_easter", "easter_window",
  "td_p", "score_100"
)]

For SEATS candidates with user regressors, X-13 needs future regressor values over its forecast horizon. seasight pads pulse-type holiday regressors with zeros for this purpose. For non-pulse regressors, such as trading-day counts, provide future calendar values yourself where possible.

Inspecting the final decision

The candidate table and the HTML report show which calendar option was selected. The final “Top candidates” table includes a “TD regressor” column with either the candidate name, such as diwali, or none.

sa_report_html(
  y = y,
  td_candidates = holiday_xreg,
  include_easter = "auto",
  title = "AirPassengers calendar-regressor example",
  outfile = tempfile(fileext = ".html")
)

This makes the calendar decision auditable: a reviewer can see whether no calendar regressor was used, whether Easter was included, and which user-supplied holiday candidate won.