I don't know why some Buddhist people are ungrateful to the people who help the Buddha constructing the Buddhism and Tipitaka.
Mendicants, these four are worthy of a monument. What four? A Realized One, a perfected one, a fully awakened Buddha; a Buddha awakened for themselves; a disciple of a Realized One; and a wheel-turning monarch. These four are worthy of a monument. (AN4.247)
MaghaPuja is the very important day of the Buddhist structure. It means there is no Buddhism today if Buddha couldn't completely found MaghaPuja day.
The members of MaghaPuja meeting were the very early monks and some were the elder monks in first Buddhist council, such as 250 sariputta's followers (The Memorizers of Entire MN, KN patisambhidamagga, KN Niddesa, KN Cariyapitaka, Abhidhamma).
Sariputta was the first model of Buddhist coucil in DN Sangitisutta. Sariputta leaded the major members of first councils in MN Mahagosingasalasutta.
Venerable Mahāmoggallāna getting up from his seclusion in the evening approached venerable Mahākassapa and said; Kassapa, let us go to venerable Sāriputta, to hear the Teaching. Venerable Mahākassapa agreed and venerable Mahāmoggallāna, venerable Mahākassapa and venerable Anuruddha approached venerable Sāriputta. Venerable Ānanda saw venerables, Mahāmoggallāna, Mahākassapa and Anuruddha approaching venerable Sāriputta to hear the Teaching and venerable Ānanda approached venerable Revata and said, friend, Revata, the Great Men are approaching venerable Sāriputta to hear the Teaching, shall we too go to hear the Teaching. Venerable Revata agreed and venerable Revata and venerable Ānanda approached venerable Sāriputta.
From above compare to DN Atthakatha that...
(My conclusion, see above link for the origin) the 1st council members gave each nikāya and commentary to 5 arahanta-groups at 1st saṅgāyanā meeting cave, upāli school got vinaya-pitaka, ānanda school got dīghanikāya, etc, to recite for the received nikāya, and there were somebodies in each school memorized more than 1 nikāya, too. Then about each 100 years later their relayed students met each other school members again to recite them together, 2nd and 3rd saṅgāyanā, again. Each saṅgāyanā, they did it on the same rule in (c) and (d).:
Ānanda and his students keep to memorize DN.
Sāriputta's students keep to memorize MN.
Kassapa and his students keep to memorize SN.
Anuruddha and his students keep to memorize AN.
(The others have learned and memorized dhamma, too, but buddha said "ānanda is the best", so everyone need him in 1st saṅgayanā).
So, it is impossible that Tipitaka still appear today without MaghaPuja and Sariputta.
And there were 1000 Jatila, too. They ordained before the Buddha enlighten. They had many Magadha-householder-follower in their hands, so the Buddhism was very stable when Buddha could let Jatila be his followers.
And Sariputta and Moggallana were sons of a Magadha's village leader as well.
It means Buddha already holded entire Magadha state by letting 1000 Jatila followers and 250 Sariputta followers change their minds.
And Magagha state was the state of all three times council. Many country outside india, including Cambodia, Thailand, Srilanka, PTS, received the Pali Canons that arranged three times Buddhist councils at Magadha state by the supporting of Magadha state's people, various kings and emperor.
Those are how MaghaPuja is important for today Buddhism. Buddhism going to vanish long time ago without MaghaPuja day.
All above is from Vinaya Pitaka Mahākhandhaka by UpaliThera, the hair dresser of Sakkya dynasty who became the best Vinaya Memorizer in Buddha living time.