TzdbZoneRulesProvider currently parses tzdata rules with the assumption that the shortened Day or Month values will be used (e.g. "Mon" instead of "Monday")

According to the zic man page, which the tzdata format adheres to, the long or short name may be used:

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       IN Names the month in which the rule takes effect. Month
              names may be abbreviated.

       ON Gives the day on which the rule takes effect. Recognized
              forms include:

                5 the fifth of the month
                lastSun the last Sunday in the month
                lastMon the last Monday in the month
                Sun>=8 first Sunday on or after the eighth
                Sun<=25 last Sunday on or before the 25th

              A weekday name (e.g., Sunday) or a weekday name preceded
              by “last” (e.g., lastSunday) may be abbreviated or spelled
              out in full.

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JDK-8339644 tracking the JDK related changes