diff --git a/docs/IMPLEMENTATION_PLAN.md b/docs/IMPLEMENTATION_PLAN.md
index 38dd2d5..50c0f3c 100644
--- a/docs/IMPLEMENTATION_PLAN.md
+++ b/docs/IMPLEMENTATION_PLAN.md
@@ -1708,7 +1708,7 @@ public enum CashBookEntryType { Income = 0, Expense = 1 }
 | ✓ | K7 | Application | Service Interfaces | 2 |
 | ✓ | K8 | Application | Service Implementations | 4 |
 | ✓ | K9 | Application | Category Seeder | 1 |
-| ☐ | K10 | Application | Day Close Integration | 1 |
+| ✓ | K10 | Application | Day Close Integration | 1 |
 | ☐ | K11 | Web | Fluxor CategoryState | 4 |
 | ☐ | K12 | Web | Fluxor CashBookState | 4 |
 | ☐ | K13 | Web | CashBook UI | 3 |
diff --git a/src/Koogle.Application/Interfaces/ICashBookService.cs b/src/Koogle.Application/Interfaces/ICashBookService.cs
index 8dd7194..f349ef9 100644
--- a/src/Koogle.Application/Interfaces/ICashBookService.cs
+++ b/src/Koogle.Application/Interfaces/ICashBookService.cs
@@ -89,4 +89,12 @@ public interface ICashBookService
     /// <param name="ct">Cancellation token.</param>
     /// <returns>True if fees exist; otherwise, false.</returns>
     Task<bool> MembershipFeesExistAsync(int year, int month, CancellationToken ct = default);
+
+    /// <summary>
+    /// Creates cash book entries for monetary penalties from a closed day.
+    /// </summary>
+    /// <param name="dayId">The day identifier.</param>
+    /// <param name="ct">Cancellation token.</param>
+    /// <returns>The number of entries created.</returns>
+    Task<int> CreatePenaltyEntriesForDayAsync(Guid dayId, CancellationToken ct = default);
 }
diff --git a/src/Koogle.Application/Services/CashBookService.cs b/src/Koogle.Application/Services/CashBookService.cs
index a781c95..d62604f 100644
--- a/src/Koogle.Application/Services/CashBookService.cs
+++ b/src/Koogle.Application/Services/CashBookService.cs
@@ -246,4 +246,80 @@ public class CashBookService : ICashBookService
     {
         return await _entryRepository.ExistsMembershipFeeForMonthAsync(_clubContext.ClubId, year, month, ct);
     }
+
+    /// <inheritdoc />
+    public async Task<int> CreatePenaltyEntriesForDayAsync(Guid dayId, CancellationToken ct = default)
+    {
+        await using var context = await _contextFactory.CreateDbContextAsync(ct);
+
+        // Get penalty category
+        var penaltyCategory = await _categoryRepository.GetSystemCategoryAsync(_clubContext.ClubId, "Spielstrafe", ct);
+        if (penaltyCategory is null)
+        {
+            return 0; // No category, skip silently
+        }
+
+        // Get all monetary PersonExpenses for the day that are still open
+        var personExpenses = await context.PersonExpenses
+            .Include(pe => pe.Person)
+            .Where(pe => pe.DayId == dayId
+                && pe.ClubId == _clubContext.ClubId
+                && pe.ExpenseType == ExpenseType.Monetary
+                && pe.PersonExpenseStatus == PersonExpenseStatus.Open
+                && !pe.IsDeleted)
+            .ToListAsync(ct);
+
+        if (personExpenses.Count == 0)
+        {
+            return 0;
+        }
+
+        // Get day for booking date
+        var day = await context.Days.FirstOrDefaultAsync(d => d.Id == dayId, ct);
+        var bookingDate = day?.PostDate ?? DateTime.UtcNow;
+
+        // Group by person and create entries
+        var groupedByPerson = personExpenses.GroupBy(pe => pe.PersonId);
+        var entriesCreated = 0;
+
+        foreach (var group in groupedByPerson)
+        {
+            var personId = group.Key;
+            var personName = group.First().Person?.Name ?? "Unbekannt";
+            var totalAmount = group.Sum(pe => pe.Price);
+
+            if (totalAmount <= 0)
+            {
+                continue;
+            }
+
+            // Create cash book entry
+            var entry = new CashBookEntry
+            {
+                Id = Guid.NewGuid(),
+                ClubId = _clubContext.ClubId,
+                CategoryId = penaltyCategory.Id,
+                EntryType = CashBookEntryType.Income,
+                Amount = totalAmount,
+                BookingDate = bookingDate,
+                Comment = $"Spielstrafen - {personName}",
+                DayId = dayId,
+                PersonId = personId,
+                CreatedAt = DateTime.UtcNow
+            };
+
+            await _entryRepository.AddAsync(entry, ct);
+            entriesCreated++;
+
+            // Mark PersonExpenses as Done
+            foreach (var pe in group)
+            {
+                pe.PersonExpenseStatus = PersonExpenseStatus.Done;
+                pe.ModifiedAt = DateTime.UtcNow;
+            }
+        }
+
+        await context.SaveChangesAsync(ct);
+        return entriesCreated;
+    }
 }
diff --git a/src/Koogle.Application/Services/DayService.cs b/src/Koogle.Application/Services/DayService.cs
index e89c28b..1717400 100644
--- a/src/Koogle.Application/Services/DayService.cs
+++ b/src/Koogle.Application/Services/DayService.cs
@@ -21,6 +21,7 @@ public class DayService : IDayService
     private readonly ICurrentClubContext _clubContext;
     private readonly IMapper _mapper;
     private readonly IEmailService _emailService;
+    private readonly ICashBookService _cashBookService;
     private readonly ILogger<DayService> _logger;
 
     /// <summary>
@@ -33,6 +34,7 @@ public class DayService : IDayService
         ICurrentClubContext clubContext,
         IMapper mapper,
         IEmailService emailService,
+        ICashBookService cashBookService,
         ILogger<DayService> logger)
     {
         _contextFactory = contextFactory;
@@ -41,6 +43,7 @@ public class DayService : IDayService
         _clubContext = clubContext;
         _mapper = mapper;
         _emailService = emailService;
+        _cashBookService = cashBookService;
         _logger = logger;
     }
 
@@ -320,6 +323,21 @@ public class DayService : IDayService
         day.ModifiedAt = DateTime.UtcNow;
         await context.SaveChangesAsync(ct);
 
+        // Create cash book entries for penalties when closing
+        if (day.Status == DayStatus.Closed)
+        {
+            try
+            {
+                var entriesCreated = await _cashBookService.CreatePenaltyEntriesForDayAsync(day.Id, ct);
+                _logger.LogInformation("Created {Count} cash book entries for day {DayId}", entriesCreated, day.Id);
+            }
+            catch (Exception ex)
+            {
+                _logger.LogError(ex, "Failed to create cash book entries for day {DayId}", day.Id);
+                // Don't rethrow - cashbook failure should not fail day closing
+            }
+        }
+
         // Send protocol emails after closing (fire and forget, log errors)
         if (day.Status == DayStatus.Closed)
         {