I'll create a Next.js application with the features you've described, using JavaScript files and Tailwind CSS for styling. I'll break this down into several components and pages to keep the code organized.

Let's start by creating the necessary files:

This setup creates a Next.js application with the following features:

1. A calendar displaying market data for each day.
2. Fetching of live market data (simulated in this example).
3. Color-coding of calendar cells based on positive (green) or negative (red) market performance.
4. Light and transparent cell colors.
5. A stock/index selector above the calendar.
6. A popup modal showing detailed information when clicking on a specific day.

To run this application, you'll need to:

1. Install the necessary dependencies (next, react, react-dom, and tailwindcss).
2. Set up Tailwind CSS in your Next.js project.
3. Run the development server with npm run dev or yarn dev.

Remember that this is a simplified version and doesn't include real API calls for market data. In a production environment, you'd want to replace the mock data in fetchMarketData with actual API calls to a financial data provider.

Also, for better state management across components, you might want to consider using React Context or a state management library like Redux, especially as your application grows in complexity.

You're right, and I apologize for the oversight. I'll update the calendar component to include months, years, and weeks, making it look exactly like a real-time calendar. Let's improve the Calendar component to address these issues.

Let's update the Calendar component to address these requirements:

These updates will create a more complete and realistic calendar view with the following improvements:

1. The calendar now shows a full month view with weeks.
2. Month and year navigation has been added with previous and next month buttons.
3. The calendar layout closely resembles a real-time calendar.
4. The existing functionality for market data display has been maintained.
5. Color-coding for positive (green) and negative (red) market performance is preserved, with a more subtle, transparent appearance.
6. The calendar is responsive and visually appealing, with a grid layout for days and proper spacing.

The updated calendar component now:

• Displays the current month and year
• Shows day names (Sun, Mon, Tue, etc.) at the top of each column
• Allows navigation between months
• Properly aligns days of the week
• Maintains the market data display and color-coding
• Keeps the popup functionality for detailed day information

To further improve the user experience, you might consider adding:

1. Highlighting the current day
2. Ability to jump to a specific month/year
3. Animations for month transitions

Remember to style the calendar further to match your desired look and feel, and to integrate it properly with your backend API for real market data when you're ready to move beyond the mock data.