Hey Stefan! Sounds were added to the Tingbot libraries with a recent update. Have a look at http://docs.tingbot.com/projects/tingbot-python/en/latest/sound.html for an example.
As for formats, mp3 might work, or you'll be better off transcoding your sound effects into WAV or OGGs.
I've not used any I2C boards with the Pi so I don't know how easy that is.... the Tingbot sound libs were tested with a USB audio dongle, so that might be a good starting point.
I found this Python library for checking if a date is a national holiday - check it out: https://pypi.python.org/pypi/holidays . You can list that in a requirements.txt file to include it in a Tingbot app.