The 15th of Every Jewish Month—Connection with the Tzadikim
In the book Gates of Reincarnation, Rabbi Chaim Vital transmitted from his teacher, the Ari HaKadosh (may the memory of the tzadik be a blessing), that on the 15th day of every Jewish month, as well as on Erev Rosh Chodesh—the 29th day of the Jewish month—we can achieve a stronger connection with our tzadikim, because their souls are present in this world at that time. This is in addition to their days of Hilula—their yahrzeit.
The most appropriate time for this is Chatzot Layla—the time after midnight—which is considered the most auspicious time for our prayers to be accepted in Heaven.
One should give tzedakah, light a 24-hour candle for all the tzadikim, and ask them to elevate our prayers as high as possible.
(Rabbi Chaim Vital “Gates of Reincarnation,” Introduction 38)