Post today's holiday to the specified channel in Slack.

• Add a new Incoming WebHook integration in Slack
• Copy your unique webhook URL to your clipboard
• Specify a channel where the message will be posted to
• Press the Add Integration button
• Paste the unique webhook URL in the curl request (line 41 of index.php)
• Also set your timezone accordingly (line 6 of index.php)
• Set up a cron job somewhere to run index.php every day
• Celebrate when the message arrives

The list of holidays came from a post on StudentBeans.com.