diff --git a/src/excel_reader.py b/src/excel_reader.py
index a84fb52..5fe2227 100644
--- a/src/excel_reader.py
+++ b/src/excel_reader.py
@@ -139,6 +139,30 @@ def get_sheet_names(file_path: str | Path) -> List[str]:
     return names
 
 
+def resolve_sheet_name(
+    sheet_name: str,
+    workbook_sheets: List[str],
+    month_mapping: Dict[str, str],
+) -> str:
+    """Resolve a month alias to the actual sheet name present in the workbook.
+
+    If *sheet_name* exists in *workbook_sheets*, it is returned unchanged.
+    Otherwise all aliases in *month_mapping* that map to the same month number
+    are checked, and the first one present in *workbook_sheets* is returned.
+    If no alias matches, the original *sheet_name* is returned so that the
+    normal missing-sheet error can fire.
+    """
+    if sheet_name in workbook_sheets:
+        return sheet_name
+    target_month = month_mapping.get(sheet_name)
+    if target_month is None:
+        return sheet_name
+    for alias, month_num in month_mapping.items():
+        if month_num == target_month and alias in workbook_sheets:
+            return alias
+    return sheet_name
+
+
 def extract_persons(
     file_path: str | Path,
     sheet_name: str,
@@ -158,6 +182,7 @@ def extract_persons(
 
     wb = _open_workbook(Path(file_path))
     try:
+        sheet_name = resolve_sheet_name(sheet_name, wb.sheetnames, config["month_mapping"])
         data = _sheet_data(wb, sheet_name)
     finally:
         wb.close()
@@ -217,6 +242,7 @@ def extract_attendance(
 
     wb = _open_workbook(Path(file_path))
     try:
+        sheet_name = resolve_sheet_name(sheet_name, wb.sheetnames, month_mapping)
         data = _sheet_data(wb, sheet_name)
     finally:
         wb.close()
diff --git a/src/validators.py b/src/validators.py
index 4297bdd..7eeb01b 100644
--- a/src/validators.py
+++ b/src/validators.py
@@ -14,6 +14,7 @@ from pathlib import Path
 from typing import Any, Dict, List, Optional
 
 from .config_loader import load_config
+from .excel_reader import resolve_sheet_name
 from .exceptions import ConfigurationError
 
 
@@ -71,18 +72,19 @@ def validate(
             )
             sheet_names = wb.sheetnames
 
-            # Check 4: sheet name valid
+            # Check 4: sheet name valid (resolve alias, e.g. Mär → Mrz)
             if sheet_name not in config["month_mapping"]:
                 errors.append(
                     f"Sheet-Name '{sheet_name}' nicht in month_mapping der config.json."
                 )
-            if sheet_name not in sheet_names:
+            resolved = resolve_sheet_name(sheet_name, sheet_names, config["month_mapping"])
+            if resolved not in sheet_names:
                 errors.append(
                     f"Sheet '{sheet_name}' existiert nicht in der Arbeitsmappe. "
                     f"Verfügbar: {sheet_names}"
                 )
             else:
-                ws = wb[sheet_name]
+                ws = wb[resolved]
                 rows = list(ws.iter_rows(values_only=True))
 
                 name_col: int = config["excel_name_column"]
@@ -179,11 +181,12 @@ def _extract_samples(
 
     try:
         wb = openpyxl.load_workbook(str(file_path), read_only=True, data_only=True)
-        if sheet_name not in wb.sheetnames:
+        resolved = resolve_sheet_name(sheet_name, wb.sheetnames, config["month_mapping"])
+        if resolved not in wb.sheetnames:
             wb.close()
             return result
 
-        ws = wb[sheet_name]
+        ws = wb[resolved]
         rows = list(ws.iter_rows(values_only=True))
         wb.close()