Add Periodic Notification blueprint

Sends notifications at specified times every N days when a control
flag is enabled. Supports multiple notification targets, flexible
time scheduling, and optional day interval with configurable start date.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Common/Periodic Notification/README.md

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+# Periodic Notification
+
+Sends notifications at specified times on a repeating day schedule, controlled by a toggle entity.
+
+## Features
+
+- Multiple notification targets (notify entities)
+- Customizable multiline message
+- Flexible time schedule (multiple times per day)
+- Configurable day interval (every N days)
+- On/off control via input_boolean or switch entity
+
+## How It Works
+
+1. The automation triggers at each configured notification time (checked every minute)
+2. It verifies the control flag entity is ON
+3. It calculates whether today falls on the correct day in the interval cycle, counting from the configured start date
+4. If all conditions pass, the message is sent to all notification targets
+
+### Day Interval Calculation
+
+The day interval uses a **start date** as a reference point. Notifications fire every N days from that date. For example, with a start date of January 1 and a 3-day interval, notifications fire on January 1, 4, 7, 10, and so on.
+
+If no start date is set, the day interval is ignored and notifications are sent every day at the configured times.
+
+Setting the day interval to **1** sends notifications every day.
+
+## Configuration
+
+| Input | Description |
+|---|---|
+| **Notification Targets** | One or more `notify` entities (e.g., `notify.mobile_app_phone`) |
+| **Message** | The notification text to send (supports multiline) |
+| **Notification Times** | Comma-separated times in 24-hour `HH:MM` format (e.g., `08:00, 14:30, 20:00`) |
+| **Day Interval** | Number of days between notification cycles (1 = daily) |
+| **Start Date** | Reference date for the interval calculation (optional — leave empty to send every day) |
+| **Control Flag** | `input_boolean` or `switch` entity to enable/disable notifications |
+
+### Time Format
+
+Times must use 24-hour format with leading zeros:
+
+- `08:00` (not `8:00`)
+- `14:30`
+- `20:00`
+
+### Debug Mode
+
+Set the `is_debug` variable to `true` in the automation YAML to enable persistent notification logging for troubleshooting.
+
+## Author
+
+Alexei Dolgolyov (dolgolyov.alexei@gmail.com)