Does any modern Buddhist tradition use a Gregorian holiday schedule? Have any modern Buddhist organization adopted the Gregorian calendar for celebrating Buddhist holidays?

Last time I tried to find something simple as Vesak or some of the other common Buddhist Holiday's, they are celebrated on a lunar calendar and differ from country to country, ref: http://en.wikipedia.org/wiki/Vesak#Dates_of_observance 

I'm not affiliated with any particular organized sangha, so I have two challenges-- deciding which holidays to celebrate, and even after that, there isn't a lot of agreement on how to calculate the day.
I've had a look around and I can't see actual Buddhist holidays that follow a Gregorian calendar. They all seem to shift around with lunar cycles. The Thai national calendar is a renumbered Gregorian calendar but the Buddhist festivals within that are lunar

However I practice with the Triratna Buddhist community in the UK and we have over a number of years celebrated 'Gregorian' festivals. For instance we have a lovely Buddhist ritual on winter solstice and then again on New Years Eve. I'm not sure if this quite answers your question and I appreciate that this isn't traditional or even standard but I think it's interesting that we are repurposing these days as Buddhist festivals just as Christians repurposed pagan festivals for Christmas and Easter.

So perhaps there are no Gregorian festivals (that I know of) right now but 'Western' Buddhist communities will be busily repurposing existing Gregorian festivals just like we are. Just a thought really.