Eeeek, you're a polite and sensitive soul aren't you?
Okay, I find that pagans generally don't mind if someone wishes them a Happy Whatever from THEIR path - regardless of whether the person they're saying it to celebrates it. I wish people a happy Pomonalia, Floralia, etc even while knowing that they don't follow my path, but it doesn't matter, you're wishing someone the blessings of that day, and that's always very acceptable
So try not to get too hung up on what every other Pagan celebrates, because, frankly, you - and everyone else on the planet - won't manage to remember who does what when
I mix and match festivals to be honest. The Romans were big on festival days, around two a month, when everything stopped, games were held, commerce suspended etc, and fasting took place. Not really going to happen in 21st C Scotland
So I celebrate festivals that have meaning to me.
I do follow most of the common "pagan markers" of the so-called "Wheel of the Year":
Starting at Imbolc (why then? Simply because it's when
I think of it as the new year. The year to me is a cycle, circular, and there's no "start" date to it

)
Minerva's birthday - 19th February - day before mine
Greater Quinquatrus on March 19th (for Minerva)
Spring Equinox (I don't call it Ostara) I celebrate because it's the balance shifting towards longer days.
Beltane at the end of April, the start of Summer. One of my favourite, favourite festivals. I also celebrate the Floralia then.
The first week in June I light the fire for the Vestalia
Midsummer, speaks for itself. Solstice
Lammas - start of August, the beginning of the Harvest.
13th August, the Vertumnalia
September 1st, Festival for Juno, Goddess of Marriage
1st November, the Pomonalia
And then Yule. Midwinter. Solstice. I also celebrate the Saturnalia round then.
So, you see, quite a busy year, but not one that is shared in every respect with other Pagans, so I don't expect others to know every date, just as I don't know all dates for other paths too