Monday, April 30 after Maghrib
It is narrated that the Holy Prophet ﷺ said, "If it be the eve of mid-Shaʿbān, then stand its night and fast its day." The report continues, "For indeed, Allah descends therein at the time the sun sets to the lowest heaven, then says, 'Is there none seeking forgiveness, that I may forgive him? Is there none seeking provision, that I may provide him? Is there none afflicted, that I may relieve him? Is there none...' and so forth, until the onset of dawn."(1)
For more information on mid-Shaʿbān, please see the following link: http://tiny.cc/MidShabanInfo. Please also see the below document for a supplication that, although not specifically from the Sunnah, has been the regular practice of some of the pious on this blessed night. One would do well to recite the supplication after maghrib, to fast on Tuesday, and to attend the maghrib, ʿishāʾ, and fajr prayers in congregation.
May Allah ﷻ bless us all in this month, and enable us to reach Ramaḍān.
(1) Recorded by ibn Mājah. Some scholars mention that the meaning of Allah's 'descent' is His opening the door of acceptance to His slaves.