When do people most probably wear T-shirts, according to the speaker?

For many years, T-shirts were simple short-sleeved undershirts for men and boys. T-shirts used to be of one colour—white. And since they were worn under shirts, they were generally not seen.
   Today things have changed a lot. T-shirt has become colourful and fashionable. It can be seen almost everywhere and on everyone. Women and children wear T-shirts as well as teenagers, university students and men from all walks of life. T-shirts are worn on the playground, at the beach or in town. They can also be worn for work. Yet, T-shirts remain relatively inexpensive and long wearing, as well as easy to care for. Smart but comfortable and convenient to wear, they have become one of the newest ideas on fashion over the world.
   Although T-shirts are now available in a wide variety of bright materials and styles, the most popular kind is the traditional cotton T-shirts with a slogan or a picture printed on the front. A T-shirt may bear a single word; a popular phrase, pictures of sportsmen, or an advertisement. As T-shirts are becoming more and more popular, new designs are coming up all the time.