Add day of week to energy chart tooltips (#29803)

* Add day of week to energy chart tooltips

* New localization helpers

src/common/datetime/format_date.ts

@@ -210,3 +210,39 @@ const formatDateWeekdayShortMem = memoizeOne(
       timeZone: resolveTimeZone(locale.time_zone, serverTimeZone),
     })
 );
+
+// Mon, Aug 10
+export const formatDateWeekdayVeryShortDate = (
+  dateObj: Date,
+  locale: FrontendLocaleData,
+  config: HassConfig
+) =>
+  formatDateWeekdayVeryShortDateMem(locale, config.time_zone).format(dateObj);
+
+const formatDateWeekdayVeryShortDateMem = memoizeOne(
+  (locale: FrontendLocaleData, serverTimeZone: string) =>
+    new Intl.DateTimeFormat(locale.language, {
+      weekday: "short",
+      month: "short",
+      day: "numeric",
+      timeZone: resolveTimeZone(locale.time_zone, serverTimeZone),
+    })
+);
+
+// Mon, Aug 10, 2021
+export const formatDateWeekdayShortDate = (
+  dateObj: Date,
+  locale: FrontendLocaleData,
+  config: HassConfig
+) => formatDateWeekdayShortDateMem(locale, config.time_zone).format(dateObj);
+
+const formatDateWeekdayShortDateMem = memoizeOne(
+  (locale: FrontendLocaleData, serverTimeZone: string) =>
+    new Intl.DateTimeFormat(locale.language, {
+      weekday: "short",
+      month: "short",
+      day: "numeric",
+      year: "numeric",
+      timeZone: resolveTimeZone(locale.time_zone, serverTimeZone),
+    })
+);

src/panels/lovelace/cards/energy/common/energy-chart-options.ts

@@ -28,6 +28,8 @@ import {
   formatDateMonthYear,
   formatDateShort,
   formatDateVeryShort,
+  formatDateWeekdayShortDate,
+  formatDateWeekdayVeryShortDate,
 } from "../../../../../common/datetime/format_date";
 import { formatTime } from "../../../../../common/datetime/format_time";
 import type { ECOption } from "../../../../../resources/echarts/echarts";
@@ -222,7 +224,9 @@ function formatTooltip(
   if (suggestedPeriod === "month") {
     period = `${formatDateMonthYear(date, locale, config)}`;
   } else if (suggestedPeriod === "day") {
-    period = `${(showCompareYear ? formatDateShort : formatDateVeryShort)(date, locale, config)}`;
+    period = showCompareYear
+      ? formatDateWeekdayShortDate(date, locale, config)
+      : formatDateWeekdayVeryShortDate(date, locale, config);
   } else {
     period = `${
       compare