Here are the details, from the Starbucks web page.
Join the movement. On April 15th, bring a reusable travel mug into your local Starbucks and get a free brewed coffee.
One person can save trees, together we can save forests.
For the good of the planet, Starbucks is encouraging everyone to switch from paper cups to reusable travel mugs. One day in March thousands of New Yorkers made the switch. Join them now by taking a pledge to do the same.
This global movement is taking place in locations around the world including East China, Taiwan, Hong Kong, Australia, Indonesia, Korea, Malaysia, New Zealand, Philippines, Singapore, Thailand, Austria, Bahrain, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Egypt, France, Germany, Greece, Jordan, Kuwait, Lebanon, Netherlands, Oman, Poland, Portugal, Qatar, Romania, Russia, Saudi Arabia, Spain, Sweden, Switzerland, Turkey, United Arab Emirates, UK, Ireland, Argentina, Bahamas, Brazil, Chile, Mexico, Peru, Puerto Rico.
Offer varies by country, ask your barista for details.