Poonam
What it is
Poonam is the full moon: the last tithi of the bright fortnight, when the moon stands complete opposite the sun and the night is at its brightest. Like Amaavas, it comes once a month and the calendar marks it as a simple lunar landmark rather than a festival, though several genuinely major Pushtimarg and Hindu observances happen to land on a Poonam — Guru Purnima, Sharad Purnima, Holi's Dolutsav — and those get their own separate entry when the day carries a real story of its own. A bare "Poonam" on the calendar with no other name attached is just that: the moon at its fullest, nothing more specific claimed for it.
Some months also carry a named Poonam tied to that month, such as Chaitri Poonam (the full moon of Chaitra), which closes out the spring month before Vaishakh begins.
Why we observe it
A Vaishnav gains from simply knowing a Poonam is there: it is the natural high point of the fortnight, the night everything the moon has been building toward for two weeks is visible at once. There is no special vrat attached to an unnamed Poonam, but many Vaishnavs use a full-moon night for a fuller darshan, a longer kirtan sitting, or simply pausing to notice the light.
The deeper bhav, if one is looked for, is the mirror image of Amaavas: fullness is not permanent either. The moon that is complete tonight will begin waning tomorrow, exactly as the moon that vanished on Amaavas began waxing the very next day. Seva kept steady through both the full nights and the dark ones, without either one being treated as more "real" than the other, is closer to how Pushtimarg actually asks a Vaishnav to live.