It is said that, we should not touch or give water to Tulsi or Peepal tree on Sunday. What is the reason behind it and what happens if someone does it?
This faith is profound in many hindu followers. It is belief that Tulsi is Lord Vishnu's beloved. so, most followers especially Vaishnavas strictly follow rituals while doing prayer (archana) of Lord Vishnu on daily basis.
NA CHINDYAT TULASIM VIPRA DVADASYAM VAISNAVAH KVACIT || vishnudharmottara purana.
Oh brahmanas, a Vaisnava never picks Tulasi leaves on Dvadasi, (the day after Ekadasi).
For some specifics days in Garuda Purana
BHANUVARAM VINA DURVAM TULASIM DVADASIM VINA JIVITASYA AVINASAYA NA VICINVITA DHARMA VIT || Garuda Purana
A scripturally learned person, if he does not want to decrease his duration of life, he should not pick Durva grass on Sundays or Tulasi leaves on the Dvadasi day.
From Padma Purana
DVADASYAM TULASI PATRAM DHATRI PATRAS CA KARTIKE LUNATI SA NARO GACCHET NIRAYAN ATI GARHITAN || from PADMA PURANA conversation between Lord Krsna and Satyabhama
Any person who picks Tulasi leaves on Dvadasi or Amalaki leaves inKartika month will fall into hell.
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As far as the Peepal Tree (aśvattha) is considered, not only on Sundays, but it shouldn't be touched on any other day except on Saturdays.
This prescription follows from the Skanda Mahā-Purāṇa 220.127.116.11
Chapter 3, Kārttikamāsa-māhātmya, Vaiṣṇava-khaṇḍa, Skanda-purāṇa
अश्वत्थपूजा स्पर्शेन कर्त्तव्या शनिवासरे ।
अन्यवारेऽश्वत्थसंगाद्दरिद्रो जायते नरः ॥ ४८ ॥
- The worship of Aśvattha should be conducted on Saturdays by touching it. On other days if a man touches it he will become poor.
English Translation by G.V. Tagare
So, the Skanda Purāṇa forbids touching the Peepal tree on all days except Saturday, when it must be duly touched and worshipped.
And the simple reason for not touching it may be quoted as - "because the scriptures say so, doing it the person becomes poor."