This lecture introduces the concept of the 'signal of the day', showcasing real-life applications of signal processing through the analysis of daily mean temperature measurements recorded by Johann Wolfgang Von Goethe in Vienna. The instructor explains the characteristics of digital signals, the computation of moving averages, and the recursive procedure to calculate them. The lecture concludes with the application of a moving average to the temperature signal, revealing a long-term trend of increasing temperatures, which serves as an illustration of global warming.