My take on the issue
I can only comment in a broader perspective on 'special days', which may go across all Traditions.
On what principles is the calendar defined?
From the early days of Buddhism, it is the common practice where the faithful 4 Assemblies of the Sangha, Bhikshus, Bhikshunis, Upasakas, Upasikas or sometimes depending on availablity and resources, part of the Fourfold assembly, have congregated together to observe and practice the Buddha Dharma and even later commemorating various days of Upavasatha (Upasatha/Uposatha), Buddhas/Bodhisattva/Dharma Protectors/Patriarch/Lineage Gurus days.
It is also known that such observances are a good and common methodology of uniting and fostering friendship and goodwill in communities and specifically, the Buddhist Sasana, both at national and domestic levels. One good example is the International Vesak Day which is a national holiday in my country, both for Buddhists and non-Buddhists. And according to some, days of solemnities and commemorations were also fixed after common consensus amongst the Sangha and community elders, besides that which is mentioned in Scriptures.
3. Calender Technicalities
Most Buddhist calender celebration events are based on Lunar Year cycles and even in most of Mahayana/Vajrayana canonical Sutra Pitaka, the commemorative dates given are based on Lunar calculations. Even then, it differs from one community to another when it comes to the usage of the Lunar Calender: there are the Indian, Sri Lankan, Chinese, Tibetan, South East Asian lunar calculations and calender. So far that I have known, in Malaysia, the Theravada and Mahayana communities have synchronised their Lunar Calenders to facilitate the Upavasatha/Uposatha Days (New and Full Moon Days, commonly known as the 1st and 15th days of the month).
4. Sutra Pitaka
Certain Sutras have cited certain days of observances to be followed.
One example is the Ullambana Day celebrated on every 15th Day of the 7th Month during the Sangha's Pravarana (Pavarana) as one can see in this here:
"The Buddha told Maudgalyayana, "The fifteenth day of the seventh month is the Pravarana Day for the assembled Sangha of the ten directions. For the sake of fathers and mothers of seven generations past, as well as for fathers and mothers of the present who are in distress, you should prepare an offering of clean basins full of hundreds of flavors and the five fruits, and other offerings of incense, oil, lamps, candles, beds, and bedding, all the best of the world, to the greatly virtuous assembled Sangha of the ten directions.
The Buddha replied "Good indeed! I am happy you asked that question. I just wanted to speak about that and now you have also asked about it. Good man, if Bhikshus, Bhikshunis, kings, crown princes, great ministers, great officials, cabinet members, the hundred ministers, and the tens of thousands of citizens wish to practice compassionate filial conduct, for the sake of the parents who bore them, as well as for the sake of fathers and mothers of seven lives past, on the fifteenth day of the seventh month, the day of the Buddha's Delight, the day of the Sangha's Pravarana, they all should place hundreds of flavors of foods in the Ullambana basins, and offer them to the Pravarana Sangha of the ten directions.
How do these principles magnify the (karmic?) results of actions?
Well, as those are originally intended to be wholesome, so also when keeping the original wholesome content and spirit of the observances, they will reap the wholesome merits and develop wholesome essential virtues.
An example from the quoted Sutra as above:
"On that day, all the holy assembly, whether in the mountains practicing dhyana samadhi, or obtaininging the four fruits of the Way, or walking beneath trees, or using the independence of the six penetrations to teach and transform Sound Hearers and Those Enlightened to Conditions, or provisionally manifesting as Bhikshus when in fact they are Great Bodhisattvas on the Tenth Ground-all complete with pure precepts and ocean-like virtue of the holy Way--should gather in a great assembly and all of like mind receive the Pravarana food.
"If one thus makes offerings to these Pravarana Sanghans, one's present father and mother, parents of seven generations past, as well as the six kinds of close relatives will escape from the three paths of suffering, and at that time attain release. Their clothing and food will spontaneously appear. If the parents are still alive, they will have wealth and blessings for a hundred years. Parents of seven generations past will be born in the heavens. Transformationally born, they will independently enter the celestial flower light, and experience limitless bliss."
The Buddha told all the good men and good women, "Those disciples of the Buddha who cultivate filial conduct should in thought after thought, constantly recall their present fathers and mothers when making offerings, as well as the fathers and mothers of seven lives past, and for their sakes perform the offering of the Ullambana basin to the Buddha and the Sangha and thus repay the loving kindness of the parents who raised and nourished them."
Would these principles apply universally to everyone, or just Vajrayana Buddhists?
I should think they may apply to everyone, with additional reasons perhaps, not mentioned here.
Namo Amitabha Buddha!