Valentine's day: February 14th, dedicated to St Valentine, romanticism and wild monkey sex.
History: The origins of St Valentine himself are a little confused but he was one of three martyrs from the third or fourth century. The notion of celebrating St Valentine's day began in the middle ages on account of how Feb 14th (the date the church associated with that particular saint) was more or less the time when birds began mating rituals and since they co-incided nicely, he became the saint associated with lurve and wild monkey sexing.
What it is not: An arbitrary day dreamed up by card companies, unless card companies existed in the mid to late middle ages, I lack research resources there but the lack of printing pressess and distribution networks suggest that they didn't.
What it is meant to be: The one day of the year where you make an extra special effort to be awesome to your significant other, or maybe to FIND that significant other. Sure, you should be awesome to your lover the whole year round but time and work and stress can make that difficult and theres no harm in having one day a year to remind them and yourself that you love them.
(references - catholic encyclopedia and the catholic forum. See also the Parliament of Fowls by Chaucer - the first known connection between Valentine's day and romantic feeling way back in 1382. That version is translated into modern english)
Thanksgiving: The date varies from nation to nation but late november as a rule. Dedicated to giving thanks (to god) for the things one has at the end of the harvest season.
History: Despite the tedious angst that emanates from America every god damned year about this holiday it is not unique to the states though it has an emphasis there that is lacking elsewhere. The specific American version (as distinct from the harvest thanksgivings in europe) originates in 1621 when 90 indians and 52 colonists got together to celebrate the harvest thanksgiving for not presumably having the resources to avoid starving to death the following winter. At this point it sort of became its own unique holiday, distinct from the version practiced elsewhere.
In a sense, the point of this holiday is a kind of yearly retrospective, the sort of thing we now attach to the new year where you look back and go "Hey God, thanks for the following things that were pretty awesome about the past year....."
What it is not: Some kind of goddamned "hurrah for massacres!" day celebration no matter how much white man's burden you pile onto it. Yes, early Americans killed an assload of Indians but they did it 365 days a year - no one gets up on monday morning and goes "God damn, I hate mondays, stupid anniversary of the massacre at buffalo ridge!". Every nation in the world has crapped on some other indiginous people at some point - the picts were massacred by the celts, who were slaughtered by the normans, who were butchered by the saxons and so on and so forth ad goddamned nauseum. Eventually the aliens will arrive and massacre the saxons. The British massacred *everyone* all over the place, it was their thing for a long time but never feel the need to attach guilt over a specific incident to a particular day. It isn't even like the fifty two colonists turned around and shot the indians either - there is no particular incident to attach shame to the day.
What it is meant to be: What it was from the start, a day when you reflect on all the things you have instead of the things you lack or want and realise how lucky you are and how much worse things could be, and maybe consider trying to improve the lot of those less fortunate. Its about family, fraternity and togetherness.
I'll come back to this theme but suffice it to say for now, if you don't like a holiday - STFU and stop trying to ruin it for those of us who can take joy in them without attaching any baggage. It isn't the holiday that is at fault, it is the arseholes around you, so take it out on them.