Christmas contractions

Why is it that the only time you hear ‘twas (‘twas the night before Christmas...”) and ‘tis (‘tis the season...) is at Christmas? After much debate and some googling, it was discovered that both are perfectly good contractions through out the year - “it was” and “it is”.

So resolution #1: At some point this year you will hear me say, “’twas a wonderful fireworks display” and “’tis time to get ready for Labor Day”

You know this seemed like a good idea, but ’twasn’t.