Enhance calendar details and reuse information

Updated descriptions of Sidereal and Tropical years, added information on reusing yearly calendars.

Author: Computer-Tsu

Calendar.md

@@ -2,10 +2,27 @@ Julian to Gregorian (11 days missing)
 in cathlic countries, October 5th 1582 through October 14th 1582 were skipped over.
 England (and some of North America) correcting the calendar skipped September 3, 1752 through Sept 13th 1752.
 <p>
-  Sidereal Year
-  365 days, 6 hours, 9 minutes, 10 seconds
+  Sidereal Year(back to the same place)<br>
+  365 days, 6 hours, 9 minutes, 10 seconds<br>
   365.2563657 days = 31,558,150 sec.
-
-  Tropical Year
-  365 days, 5 hours, 48 minutes, 45 seconds
+</p>
+<p>
+  Tropical Year (back to the same season) <b><i>What we use</i></b><br>
+  365 days, 5 hours, 48 minutes, 45 seconds<br>
   365.2421875 days = 31,556,925 sec
+</p>
+
+## Reusing a yearly calendar
+
+A printed calendar can often be reused in a later year if the **month layout matches exactly**: the same dates fall on the same weekdays. In other words, if January 1 lands on the same weekday in both years, and one year is not “disturbed” by a leap-day difference in the wrong place, the whole calendar page sequence can line up again.
+
+For many calendars, the first repeat happens after **6 or 11 years**. That’s because the Gregorian calendar shifts by one weekday each common year, and by two weekdays after a leap year, so the pattern eventually cycles back into place. Calendars for **leap years** often repeat after **28 years**, because the leap-year pattern itself repeats on a 28-year cycle in the Gregorian system.
+
+In practical terms, if you keep a wall calendar or desk calendar and only care about the **month, date, and day of week**, you do not need to wait a full generation to reuse it. Some years will match again surprisingly soon, while others take longer. The exact reuse year depends on whether the original year was a common year or a leap year, and on how the leap-day spacing falls around it.
+
+A simple rule of thumb is:
+
+- **Common-year calendars:** often reusable after **6** or **11** years.
+- **Leap-year calendars:** often reusable after **28** years.
+
+So a calendar bought for one year may come back into service much sooner than people expect. If you know the original year, you can usually find the next matching year with a quick weekday check.