Bhishmashtami falls on January 25, 2018.
It is the day after Rathasapthami. It comes on the Shukla Paksha Ashtami in Thai month.
Bhishma was the grandsire in Mahabharata, the great-grandfather of Kauravas and Pandavas and he accepted death in the Kurukshetra battlefield by siding Kauravas. He is the son of Mother Ganges and king Santanu and is the warrior unbeatable for his valour.
With his boon for inviting death any day as per his wish, Bhishma waited on the arrow bed for the Uttarayan Kalam when the Sun would enter Capricorn to start His northward journey. On the eight day in the Shukla Paksha from Amavasya in the month of Magi(Thai), Bhishma embraced death. It is the Ashtami thithi which has been observed as Bhishmashtami every year.
Performing tarpanam with sesame seeds and water is considered very special today. It is by way of paying homage to this great soul and also to one’s ancestors. People pray with this saying,
"May Bhishma, the son of Santanu, the speaker of truth and defeater of his passions, obtain by this water the oblations due by sons and grandsons."
By doing tarpanam on this day, people get rid of their past sins and cherish the fruits of their good deeds. Childless people fast on Bhishmashtami to get a child and offer tarpanam to Bhishma on this day. If the Ganges water is offered by way of tarpanam today, it is considered as the most auspicious.
Bhishma is the great hero who is the embodiment of loyalty and selflessness. Remembering him on this day with due respect (by way of oblation) will surely bring spiritual benefits to us.