accomodated the suggestions

Author: venom1204

NEWS.md

@@ -333,7 +333,7 @@
 
 19. Ellipsis elements like `..1` are correctly excluded when searching for variables in "up-a-level" syntax inside `[`, [#5460](https://github.com/Rdatatable/data.table/issues/5460). Thanks @ggrothendieck for the report and @MichaelChirico for the fix.
 
-20. `week()` now calculates the week of the year sequentially, where days 1-7 are always week 1. This fixes a long-standing bug where the first week of the year was incorrectly calculated as having only 6 days, [#2611](https://github.com/Rdatatable/data.table/issues/2611). Thanks to @MichaelChirico for the report and @venom1204 for the fix.
+20. BREAKING CHANGE: `week()` now calculates the week of the year sequentially (days 1-7 are week 1), fixing a bug where the first week could have 6 days. A one-time warning is now issued if this change affects the output for a given input, which can be disabled via `options(datatable.warn.week.change = FALSE)`. [#2611](https://github.com/Rdatatable/data.table/issues/2611). Thanks to @MichaelChirico for the report and @venom1204 for the fix.
 
 ### NOTES
 

inst/tests/tests.Rraw

@@ -18384,7 +18384,7 @@ x = c("1111-11-11", "2019-01-01", "2019-02-28", "2019-03-01", "2019-12-31", "202
 test(2236.1, yday(x), c(315L, 1L, 59L, 60L, 365L, 60L, 61L, 366L, 1L, 366L, 60L, NA))
 test(2236.2, mday(x), c(11L, 1L, 28L, 1L, 31L, 29L, 1L, 31L, 1L, 31L, 1L, NA))
 test(2236.3, wday(x), c(7L, 3L, 5L, 6L, 3L, 7L, 1L, 5L, 1L, 2L, 2L, NA))
-test(2236.4, week(x), c(45L, 1L, 9L, 9L, 53L, 9L, 9L, 53L, 1L, 53L, 9L, NA))
+test(2236.4, suppressWarnings(week(x)), c(45L, 1L, 9L, 9L, 53L, 9L, 9L, 53L, 1L, 53L, 9L, NA))
 test(2236.5, month(x), c(11L, 1L, 2L, 3L, 12L, 2L, 3L, 12L, 1L, 12L, 3L, NA))
 test(2236.6, quarter(x), c(4L, 1L, 1L, 1L, 4L, 1L, 1L, 4L, 1L, 4L, 1L, NA))
 test(2236.7, year(x), c(1111L, 2019L, 2019L, 2019L, 2019L, 2020L, 2020L, 2020L, 2040L, 2040L, 2100L, NA))
@@ -21821,3 +21821,5 @@ test(2342.1, week(as.IDate("1970-01-01") + 0:7), c(1L,1L,1L,1L,1L,1L,1L,2L))  #
 test(2342.2, week(as.IDate(c("2012-02-28","2012-02-29","2012-03-01"))), c(9L,9L,9L))  # leap day stays in same week
 test(2342.3, week(as.IDate(c("2019-12-31","2020-01-01"))), c(53L,1L))  # year boundary non-leap → reset to 1
 test(2342.4, week(as.IDate(c("2020-12-31","2021-01-01"))), c(53L,1L))  # year boundary leap → reset to 1
+test(2342.5, week(as.IDate("2021-01-06") + 0:6), c(1L,1L,2L,2L,2L,2L,2L))
+test(2342.6, week(as.IDate(c("2016-02-27","2016-02-28","2016-02-29","2016-03-01","2016-03-02"))), c(9L,9L,9L,9L,9L))

src/idatetime.c

@@ -1,5 +1,7 @@
 #include "data.table.h"
 
+static int week_warning_issued = 0;
+
 static const int YEARS400 = 146097;
 static const int YEARS100 = 36524;
 static const int YEARS4 = 1461;
@@ -143,6 +145,29 @@ SEXP convertDate(SEXP x, SEXP type)
     else if (!strcmp(ctype_str, "yearqtr")) { ctype = YEARQTR; ansint = false; }
     else internal_error(__func__, "invalid type, should have been caught before"); // # nocov
 
+    if (ctype == WEEK) {
+        if (!week_warning_issued) {
+            SEXP dt_week_warn_opt = GetOption(install("datatable.warn.week.change"), R_NilValue);
+            if (!isLogical(dt_week_warn_opt) || LOGICAL(dt_week_warn_opt)[0] != FALSE) {
+                for (int i = 0; i < n; i++) {
+                    if (ix[i] == NA_INTEGER) continue;
+
+                    int yday;
+                    convertSingleDate(ix[i], YDAY, &yday);
+
+                    int old_week = (yday / 7) + 1;
+                    int new_week = ((yday - 1) / 7) + 1;
+
+                    if (new_week != old_week) {
+                        Rf_warning("data.table::week() behavior has changed and is now sequential (day 1-7 is week 1). The first week of the year may differ from previous versions. To suppress this warning, run: options(datatable.warn.week.change = FALSE)");
+                        week_warning_issued = 1;
+                        break;
+                    }
+                }
+            }
+        }
+    }
+
     if (ansint) {
         SEXP ans = PROTECT(allocVector(INTSXP, n));
         int *ansp = INTEGER(ans);