diff --git a/apps/api/src/calendar/calendar.service.ts b/apps/api/src/calendar/calendar.service.ts
index 9472941..4ed0110 100644
--- a/apps/api/src/calendar/calendar.service.ts
+++ b/apps/api/src/calendar/calendar.service.ts
@@ -8,6 +8,9 @@ import { PrismaService } from '../prisma/prisma.service';
 import { CalendarCryptoService } from './crypto.service';
 import { CreateCalendarSourceDto } from './dto/create-calendar-source.dto';
 import { UpdateCalendarSourceDto } from './dto/update-calendar-source.dto';
+import { ICSProvider } from './providers/ics.provider';
+import { CalDAVProvider } from './providers/caldav.provider';
+import { ExchangeProvider } from './providers/exchange.provider';
 
 /**
  * Common interface for normalized calendar events across all provider types.
@@ -79,6 +82,17 @@ const PRIVATE_IP_PATTERNS = [
   /^\[fe80:/,
 ];
 
+/**
+ * In-memory event cache entry with TTL (Pitfall 4).
+ */
+interface CacheEntry {
+  events: CalendarEvent[];
+  expiresAt: number;
+}
+
+/** Cache TTL in milliseconds (5 minutes). */
+const CACHE_TTL_MS = 5 * 60 * 1000;
+
 /**
  * Service for calendar source CRUD and event aggregation.
  *
@@ -89,9 +103,15 @@ const PRIVATE_IP_PATTERNS = [
 export class CalendarService {
   private readonly logger = new Logger(CalendarService.name);
 
+  /** Per-user event cache with TTL (Pitfall 4). Key: `userId:from:to`. */
+  private readonly eventCache = new Map<string, CacheEntry>();
+
   constructor(
     private readonly prisma: PrismaService,
     private readonly crypto: CalendarCryptoService,
+    private readonly icsProvider: ICSProvider,
+    private readonly caldavProvider: CalDAVProvider,
+    private readonly exchangeProvider: ExchangeProvider,
   ) {}
 
   /**
@@ -215,25 +235,182 @@ export class CalendarService {
 
   /**
    * Test connection to a calendar source via its provider.
-   * Stub — full implementation in Task 3.
+   * Updates lastSyncAt/lastSyncError on the source record.
    */
   async testConnection(id: string, userId: string): Promise<{ success: boolean; error?: string }> {
     const source = await this.prisma.calendarSource.findUnique({ where: { id } });
     if (!source) throw new NotFoundException('Calendar source not found');
     if (source.userId !== userId) throw new ForbiddenException('Not your calendar source');
-    return { success: false, error: 'Not yet implemented' };
+
+    const provider = this.getProvider(source.type);
+    const decryptedSource = {
+      url: source.url,
+      username: source.username ?? undefined,
+      password: source.encryptedPassword
+        ? this.crypto.decrypt(source.encryptedPassword)
+        : undefined,
+      exchangeMode: source.exchangeMode,
+      id: source.id,
+    };
+
+    try {
+      const success = await provider.testConnection(decryptedSource);
+
+      await this.prisma.calendarSource.update({
+        where: { id },
+        data: {
+          lastSyncAt: success ? new Date() : undefined,
+          lastSyncError: success ? null : 'Connection test failed',
+        },
+      });
+
+      return { success };
+    } catch (error) {
+      const errorMsg = 'Connection failed'; // T-05-13: generic error, no credentials
+      await this.prisma.calendarSource.update({
+        where: { id },
+        data: { lastSyncError: errorMsg },
+      });
+      return { success: false, error: errorMsg };
+    }
   }
 
   /**
-   * Aggregate events from all visible sources for a user.
-   * Stub — full implementation in Task 3.
+   * Aggregate events from all visible sources for a user (CAL-02/CAL-03).
+   *
+   * - Loads only isVisible: true sources
+   * - Decrypts credentials per source
+   * - Fetches in parallel with Promise.allSettled (one failing source doesn't break others)
+   * - Merges + sorts by start ascending
+   * - Caches per user with 5-minute TTL (Pitfall 4)
    */
   async aggregateEvents(
     userId: string,
     from?: string,
     to?: string,
   ): Promise<CalendarEvent[]> {
-    return [];
+    const now = new Date();
+    const fromDate = from ? new Date(from) : now;
+    const toDate = to ? new Date(to) : new Date(now.getTime() + 30 * 24 * 60 * 60 * 1000); // Default: +30 days
+
+    // Check cache first
+    const cacheKey = `${userId}:${fromDate.toISOString()}:${toDate.toISOString()}`;
+    const cached = this.eventCache.get(cacheKey);
+    if (cached && cached.expiresAt > Date.now()) {
+      // Serve cached immediately, trigger background refresh if close to expiry
+      if (cached.expiresAt - Date.now() < CACHE_TTL_MS / 2) {
+        this.refreshCacheInBackground(userId, fromDate, toDate, cacheKey);
+      }
+      return cached.events;
+    }
+
+    // Fetch fresh
+    const events = await this.fetchAndCacheEvents(userId, fromDate, toDate, cacheKey);
+    return events;
+  }
+
+  /**
+   * Fetches events from all visible sources, caches them, and returns.
+   */
+  private async fetchAndCacheEvents(
+    userId: string,
+    from: Date,
+    to: Date,
+    cacheKey: string,
+  ): Promise<CalendarEvent[]> {
+    const sources = await this.prisma.calendarSource.findMany({
+      where: { userId, isVisible: true },
+    });
+
+    if (sources.length === 0) return [];
+
+    // Fetch from all sources in parallel — Promise.allSettled so one failure doesn't break others
+    const results = await Promise.allSettled(
+      sources.map(async (source) => {
+        const provider = this.getProvider(source.type);
+        const decryptedSource = {
+          url: source.url,
+          username: source.username ?? undefined,
+          password: source.encryptedPassword
+            ? this.crypto.decrypt(source.encryptedPassword)
+            : undefined,
+          exchangeMode: source.exchangeMode,
+          id: source.id,
+          color: source.color,
+        };
+
+        try {
+          const events = await provider.fetchEvents(decryptedSource, from, to);
+
+          // Update sync status on success
+          await this.prisma.calendarSource.update({
+            where: { id: source.id },
+            data: { lastSyncAt: new Date(), lastSyncError: null },
+          });
+
+          return events;
+        } catch (error) {
+          // Update sync error — generic message (T-05-13)
+          this.logger.warn(
+            `Failed to fetch events from source ${source.id} (${source.type}): ${(error as Error).message}`,
+          );
+          await this.prisma.calendarSource.update({
+            where: { id: source.id },
+            data: { lastSyncError: 'Event fetch failed' },
+          });
+          return [] as CalendarEvent[];
+        }
+      }),
+    );
+
+    // Merge all successful results
+    const allEvents: CalendarEvent[] = [];
+    for (const result of results) {
+      if (result.status === 'fulfilled') {
+        allEvents.push(...result.value);
+      }
+    }
+
+    // Sort by start ascending
+    allEvents.sort((a, b) => a.start.getTime() - b.start.getTime());
+
+    // Cache result
+    this.eventCache.set(cacheKey, {
+      events: allEvents,
+      expiresAt: Date.now() + CACHE_TTL_MS,
+    });
+
+    return allEvents;
+  }
+
+  /**
+   * Refreshes cache in the background without blocking the response.
+   */
+  private refreshCacheInBackground(
+    userId: string,
+    from: Date,
+    to: Date,
+    cacheKey: string,
+  ): void {
+    this.fetchAndCacheEvents(userId, from, to, cacheKey).catch((error) => {
+      this.logger.warn(`Background cache refresh failed: ${(error as Error).message}`);
+    });
+  }
+
+  /**
+   * Returns the provider instance for a given source type.
+   */
+  private getProvider(type: string): CalendarProvider {
+    switch (type) {
+      case 'ics':
+        return this.icsProvider;
+      case 'caldav':
+        return this.caldavProvider;
+      case 'exchange':
+        return this.exchangeProvider;
+      default:
+        throw new Error(`Unknown calendar source type: ${type}`);
+    }
   }
 
   /**