وإنما كره الصيام في أيام التشريق " لان القوم زوار الله عز وجل فهم في ضيافته ولا ينبغي لضيف أن يصوم عند من زاره وأضافه.
Fasting during the days of Tashreeq was disliked because the people are the guests of Allah سبحانه وتعالى, Mighty and Majestic, and are under His hospitality. It is not appropriate for a guest to fast while visiting and being hosted by their Host.
Commentary #9