'Tis the season

I got this Christmas card from my long-term pen pal in England. We've been writing to each other for more than 10 years now, and we met up for the first time in June 2015, when I was in England.

I didn't know that 'tis is a contracted form of it is. I've seen the phrase, 'tis the season' many times but I haven't looked it up somehow. I had thought that it would be a contraction of this instead. I should be thankful to my pen pal for bringing this phrase to me. One website also tells me that 'tis has been used by English speakers since the fifteenth century, especially in poetry, and there is another contracted form, 'twas, for 'it was.