It is the seventh day of Paryushan Parv or Dash Lakshan Dharm.
Tap means Penance; This does not only mean fasting but also includes a reduce diet, restriction of certain types of foods, avoiding tasty foods, etc. The purpose of penance is to keep desires and passions in control. Over-indulgence inevitable leads to misery. Penance leads to an influx of punya karmas. Meditation prevents the rise of desires and passions in the soul. In a deep state of meditation the desire to intake food does not arise. Our first Tirthankara, Adinath Bhagwan was in such a meditative state for six months. The only food he consumed during these six months was the happiness from within.