der Ersatzruhetag, -e

The German word "Ersatzruhetag" is a compound noun made up of:

• Ersatz = replacement, substitute
• Ruhetag = rest day, day off (from ruhen = to rest + Tag = day)

Meaning:
"Ersatzruhetag" refers to a substitute rest day—an alternative day off given, often in compensating for work done on a usual rest day or holiday.

Similar words:

• Freizeit (free time, leisure)
• Pause (break)
• Feiertag (public holiday, day off)

Difference:
While "Freizeit" and "Pause" refer generally to free time or short breaks, and "Feiertag" is a public holiday, an Ersatzruhetag specifically denotes a compensatory day off substituted for a missed regular rest day.