Verizon ETF

[AC Question]

My phone is in 2-year contract with Verizon and it's been almost 11month that I have been in it. I am planning to switch over to T-mobile. I am going to keep using my phone, which makes me responsible for paying ETF. By bill due date for Verizon is 15th of every month, but new cycle starts on the 21st of the month. If I terminate the phone now, I would have to pay $250 with tax. If I want to pay $10 less of the current ETF, do I have to terminate the contract right after the 15th or 21st?

 

[Almeuit]

The 21st.