What exactly is your idea? That no celebrations of mid-winter and spring excisted before the Christian holidays of Christmas and Easter? That they did exist but that they timing of Christmas and Easter have nothing to do with them? That they did exist but Christmas and Easter have no traces of previous pagan traditions?
That Christmas and Easter were dated for reasons quite independent of existing pagan festivals, if any, and in fact in the case of Easter for reasons that are absolutely obvious and explicit - from the Jewish Passover.
And are you asking us to prove, with pre-christian sources describing such celebrations and traditions, the case for any and all of these?
I believe there were festivals in Spring and Winter sure - but I'm not convinced they were related to Solstices, as seems ot be generally accepted post-Frazer by everyone who has not looked at the actual evidence. Obviously regional seasonal festivals might exist - but in fact my studies across a number of pagan religions have not yet established that the proposed Equinox patter or the state of the sun really mattered anywhere. In Egypt, it was the flooding of the Nile that mattered, and in Mesopotamia, the seasons are very different. Even in the Middle East we can't really apply our Spring/Summer/Autumn/Winter model. The basic problem is that we apply a temperate European model to relighions all over the place, and assume that all religious beliefs and festival cycles should follow our weather and growing patterns. They don't, and while the ancients clearly were interested in astronomy, they were more interested in celebrating certain days for totally abstract reasons, or because of local agricultural reasons, etc, etc. A classic example of a religious calendar which has nothing to do with the solstices, seasonal celebration etc,, is the ancient Jewish one. Some events are lunar, most as i recall, and the whole thing does not in any way match up to the model we impose on antiquity.
Yet this solstice thing is now almost completely unchallenged, despite a complete lack of strong evidence to support it. We can find isolated solstice celebrations sure, but no more than we might expect by chance. Yet everyone knows that "primitive man" (women were never primitive it seems

) celebrated the solstices... SO the easiest way for me to poke holes in the whole thing, and to show how pernicious and prevalent the myth has become, is to point out the Christian-Pagan examples and show how they are myths...
In summary of those
1. Easter derives from the Jewish Passover, as when Jesus was supposedly crucified. That is why it is when it is. If you want to try and make a case for the Passover being a "spring festival" feel free, but maybe look in to seasonal variations and agricultural practices in the region first?

The fact that Easter derives from Pasover, stated in the bible, and absolutely obvious, seems irrefutable.
2. We do not know Jesus was born on the 25th December, though it is at least a 1 in 365 chance, assuming (as I do) he existed. We do know why that date, became of the many dates of Christmas (I'm surprised no one has pointed out all the different Christmas that arise from different calendars and different churches choices), this one is explained by several early authors as arising from a tradition about the Creation of Adam - see Augustine On The Trinity, cited above. Owing to the early Christian theological concept of Jesus as Second Adam - explicit in the authentic Paulines - there is little reason to doubt why that arose. Furthermore the evidence for a "pagan" Sol Invictus festival on the date is one hundred and fifty years after the Christian celebration, and weak anyway for reasons discussed.
Sure there's a lot of overzealous linking of holiday traditions to flimsily evidenced pagan roots, but disregarding all of it?
Haven't really got on to specific practices, but yes, things like Easter Eggs, Well Dressing, Maypoles, Hobby Horses, Wassailing, and all this stuff popularly assumed to be pagan survivals -- I'm afraid not. Osme of it comes from those traditions of the Medieval Church we call Sacramentals (not Sacraments), and much is secular, but hardly any of it is more than a few centuries old. IF we look at specific cases I can show by example...
It's fairly clear that there was a Yule celebration around mid-winter before Christianity. The Viking kings didn't just baptize the country by threat of death and then invent a new word for the celebration of the nativity. It's also fairly clear that ancestor worship was still a part of Scandinavian culture at the time, and that this survived in the form of offerings of food to the spirits and supernaturals of the farm on feast days. Today only setting out porridge to the "fjøsnisse" (barn-gnome) on Christmas eve is left, and very few people do so, with the few who do being influenced by and confused by the non-Scandinavian, and much younger tradition of putting out cookies for Santa (Julenissen - the Yule gnome).
Godt NytÅr! Most scholars of Scandianian pre-Christian religious practice place it originally as part of the Winter Nights, in October, the term being adopted for Christmas. I can dig out quite a lot on this I think if you are interested. I think my dad might well put out porridge - I'll have to ask him - his stories of Christmas in Danmark in the twenties and thirties are always worth hearing.

Mind you I have an
Ars Magica project to complete today, so I'd better get back to work...
cj x