Serafe's website says that they randomly assign households to one of twelve groups. The first group is invoiced annually in January, the second in February, etc.

If you're in Group 1, you'll receive a bill for CHF 365 in January 2019 to cover 2019. Your next bill will arrive in January 2020, to cover the fee to the end of 2020.

If you're in Group 2, you'll receive a bill for CHF 365/12 in January 2019 to cover the fee for January 2019. Then you'll receive a bill in February, for the fee to the end of January 2020. Your next bill will arrive in February 2020, for the fee up to the end of January 2021.

If you're in Group 3, you'll receive a bill for CHF (365/12)*2 in January 2019 to cover the fee for January and February 2019. Then you'll receive a bill in March, for the fee to the end of February 2020. Your next bill will arrive in March 2020, for the fee up to the end of February 2021.

And so on...

Note, you can opt to pay the fee quarterly, without additional charge if you do it electronically. If you ask for a hard copy invoice, you'll pay an extra CHF 2.00 per quarter.