Many devotees of Mahadev worship Him in Savan by offering Bilwa leaves (बेल पत्र). However, there are certain rules which must be adhered to while plucking Bael leaves. I consider it fit to tell them before Savan starts. RT and also inform them to your friends, relatives. 1/7

Most important thing is plucking set of 3 leaves (see image below) one at a time. NEVER break the entire branch. To save their labour, people often break entire branch of Bael Tree and then pluck leaves one by one. Never break the branch at once to save your hard work. 2/7

Second, recite the following Mantra before breaking leaves:

अमृतोद्भव श्रीवृक्ष महादेवप्रिय: सदा।
गृह्यामि तव पत्राणि शिवपूजार्थमादरात्॥

Through this mantra, one takes pledge to pluck Bael Patra 'only' to worship Lord Shiva. This mantra does not permit breaking the branch. 3/7

According to Lingapurana, Bael Patra can't be plucked at any time.

अमारिक्तासु संक्रान्त्यामष्टम्यामिन्दुवासरे ।
बिल्वपत्रं न च छिन्द्याच्छिन्द्याच्चेन्नरकं व्रजेत ॥

Plucking Bael Patra is PROHIBITED on lunar dates of Chaturthi, Ashtami, Navami, Chaturdashi and Amavasya. 4/7

Plucking Bael Patra is also NOT ALLOWED on Mondays. Plucking Bael Patras on Sankranti (when Sun changes Sign which usually happens on 15-16th of every month) is also prohibited as per above mantra.

However, offering Bael Patra on these dates and days isn't prohibited. 5/7

अर्पितान्यपि बिल्वानि प्रक्षाल्यापि पुन: पुन:।
शंकरायार्पणीयानि न नवानि यदि चित्॥

Skandapurana says that if one does not have FRESH Bael Patra to offer to Lord Shiva, leaves already offered can be offered again after washing. One can wash and simply offer as many times. 6/7

So, if one cannot break Bael leaves on abovementioned dates, or does not have time to collect them, Bel Patra already offered in temple can be reused after washing them again.

Since our texts permit them, do not hesitate. A Bael Patra is extremely dear to Lord Shiva. 7/7