fix: snap initial preSelection to first enabled day in month

src/calendar.tsx

@@ -1,5 +1,4 @@
 import { clsx } from "clsx";
-import { differenceInDays } from "date-fns";
 import React, { Component, createRef } from "react";
 
 import CalendarContainer from "./calendar_container";
@@ -41,9 +40,7 @@ import {
   DEFAULT_YEAR_ITEM_NUMBER,
   getMonthInLocale,
   type Locale,
-  getStartOfMonth,
-  getEndOfMonth,
-  isDayDisabled,
+  getEnabledDateInMonth,
 } from "./date_utils";
 import InputTime from "./input_time";
 import Month from "./month";
@@ -413,29 +410,8 @@ export default class Calendar extends Component<CalendarProps, CalendarState> {
     this.props.setPreSelection && this.props.setPreSelection(date);
   };
 
-  getEnabledPreSelectionDateForMonth = (date: Date) => {
-    if (!isDayDisabled(date, this.props)) {
-      return date;
-    }
-
-    const startOfMonth = getStartOfMonth(date);
-    const endOfMonth = getEndOfMonth(date);
-
-    const totalDays = differenceInDays(endOfMonth, startOfMonth);
-
-    let preSelectedDate = null;
-
-    for (let dayIdx = 0; dayIdx <= totalDays; dayIdx++) {
-      const processingDate = addDays(startOfMonth, dayIdx);
-
-      if (!isDayDisabled(processingDate, this.props)) {
-        preSelectedDate = processingDate;
-        break;
-      }
-    }
-
-    return preSelectedDate;
-  };
+  getEnabledPreSelectionDateForMonth = (date: Date) =>
+    getEnabledDateInMonth(date, this.props);
 
   handleMonthChange = (date: Date): void => {
     const enabledPreSelectionDate =

src/date_utils.ts

@@ -1036,6 +1036,41 @@ export function isDayDisabled(
   );
 }
 
+/**
+ * Returns a selectable day in the same month as the given date.
+ *
+ * If the date itself is enabled it is returned unchanged. Otherwise the month
+ * is scanned from its first day and the first enabled day is returned. This
+ * keeps a day keyboard-focusable when the requested date (e.g. `openToDate`)
+ * falls on a disabled day, such as a gap between `includeDates`. Returns `null`
+ * when the month has no selectable day.
+ *
+ * @param date - The reference date whose month is searched.
+ * @param options - The options used to determine whether a day is disabled.
+ * @returns - The original date, the first enabled day in its month, or `null`.
+ */
+export function getEnabledDateInMonth(
+  date: Date,
+  options: DateFilterOptionsWithDisabled = {},
+): Date | null {
+  if (!isDayDisabled(date, options)) {
+    return date;
+  }
+
+  const startOfMonth = getStartOfMonth(date);
+  const endOfMonth = getEndOfMonth(date);
+  const totalDays = differenceInCalendarDays(endOfMonth, startOfMonth);
+
+  for (let dayIdx = 0; dayIdx <= totalDays; dayIdx++) {
+    const processingDate = addDays(startOfMonth, dayIdx);
+    if (!isDayDisabled(processingDate, options)) {
+      return processingDate;
+    }
+  }
+
+  return null;
+}
+
 /**
  * Checks if a day is excluded.
  *

src/index.tsx

@@ -25,6 +25,7 @@ import {
   subYears,
   isDayDisabled,
   isDayInRange,
+  getEnabledDateInMonth,
   getEffectiveMinDate,
   getEffectiveMaxDate,
   parseDate,
@@ -496,6 +497,13 @@ export class DatePicker extends Component<DatePickerProps, DatePickerState> {
           ? maxDate
           : defaultPreSelection;
 
+    // If the bounded date lands on a disabled day (e.g. openToDate points at a
+    // gap between includeDates), snap to the first enabled day in that month so
+    // a day stays keyboard-focusable. See issue #6286.
+    const safePreSelection =
+      getEnabledDateInMonth(boundedPreSelection, this.props) ??
+      boundedPreSelection;
+
     // Convert selected/startDate to zoned time for display if timezone is specified
     let initialPreSelection = this.props.selectsRange
       ? this.props.startDate
@@ -509,7 +517,7 @@ export class DatePicker extends Component<DatePickerProps, DatePickerState> {
       open: this.props.startOpen || false,
       preventFocus: false,
       inputValue: null,
-      preSelection: initialPreSelection ?? boundedPreSelection,
+      preSelection: initialPreSelection ?? safePreSelection,
       // transforming highlighted days (perhaps nested array)
       // to flat Map for faster access in day.jsx
       highlightDates: getHighLightDaysMap(this.props.highlightDates),

src/test/date_utils_test.test.ts

@@ -19,7 +19,9 @@ import {
   isSameQuarter,
   isSameYear,
   isDayDisabled,
+  getEnabledDateInMonth,
   isDayExcluded,
+  formatDate,
   isMonthDisabled,
   isQuarterDisabled,
   isYearDisabled,
@@ -346,6 +348,28 @@ describe("date_utils", () => {
     });
   });
 
+  describe("getEnabledDateInMonth", () => {
+    it("returns the date unchanged when it is enabled", () => {
+      const day = newDate("2024-06-15");
+      expect(getEnabledDateInMonth(day, {})).toBe(day);
+    });
+
+    it("returns the first enabled day in the month when the date is disabled", () => {
+      const includeDates = [newDate("2024-06-10"), newDate("2024-06-20")];
+      const result = getEnabledDateInMonth(newDate("2024-06-15"), {
+        includeDates,
+      });
+      expect(formatDate(result!, "yyyy-MM-dd")).toBe("2024-06-10");
+    });
+
+    it("returns null when no day in the month is enabled", () => {
+      const includeDates = [newDate("2024-07-01")];
+      expect(
+        getEnabledDateInMonth(newDate("2024-06-15"), { includeDates }),
+      ).toBe(null);
+    });
+  });
+
   describe("isDayExcluded", () => {
     it("should not be excluded by default", () => {
       const day = newDate();

src/test/datepicker_test.test.tsx

@@ -166,6 +166,42 @@ describe("DatePicker", () => {
     expect(instance?.state.preSelection).toBe(originalPreSelection);
   });
 
+  describe("initial preSelection when openToDate is a disabled day (issue #6286)", () => {
+    it("snaps preSelection to the first enabled day in the month so a day stays keyboard-focusable", () => {
+      const includeDates = [newDate("2024-06-10"), newDate("2024-06-20")];
+      const { instance, container } = renderDatePickerWithRef({
+        inline: true,
+        includeDates,
+        // a gap day: inside the available range but not itself selectable
+        openToDate: newDate("2024-06-15"),
+      });
+
+      // a roving-tabindex day must exist, otherwise keyboard and screen-reader
+      // users cannot move into the grid
+      expect(
+        container.querySelector('.react-datepicker__day[tabindex="0"]'),
+      ).toBeTruthy();
+
+      expect(instance?.state.preSelection).toBeTruthy();
+      expect(formatDate(instance!.state.preSelection!, "yyyy-MM-dd")).toBe(
+        "2024-06-10",
+      );
+    });
+
+    it("keeps an enabled openToDate as the preSelection", () => {
+      const includeDates = [newDate("2024-06-10"), newDate("2024-06-20")];
+      const { instance } = renderDatePickerWithRef({
+        inline: true,
+        includeDates,
+        openToDate: newDate("2024-06-20"),
+      });
+
+      expect(formatDate(instance!.state.preSelection!, "yyyy-MM-dd")).toBe(
+        "2024-06-20",
+      );
+    });
+  });
+
   it("short-circuits day key navigation when keyboard navigation is disabled", () => {
     const onKeyDown = jest.fn();
     const preSelection = newDate("2024-06-15");