Question
Why can Tulsi leaves only be picked on certain days?
Answer
This question seemed to be discussed a lot and you can find some citations from Hari Bhakti Vilasa in the link below: http://www.lotusimprints.com/new/blog/2008/12/14/qa-worship-of-tulasi-devi-on-dvadasi-17-other-questions/
The take away from the above link: Tulasi is a devotee of Lord Vishnu (Kesava priya) and she is said to practice vratam/fasting on Ekadasi and so as not to disturb her in potentially weak condition people don't pick her leaves on the following day. In some traditions days besides Ekadasi are also followed for various religious practices and the same rule seems to apply.
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