Since I see I am not the only one enticed by the 100+GB/mo pre-paid Vodafone SIM cards,
- You will get lots of text messages towards the end of the four weeks suggesting that you extend it and offering deals if you extend it early. If you extend it for another 28 days, those days do not begin when your current purchase ends.
I simply ensure that I have enough money in my Vodafone account to allow it to renew automatically.
Adding money to my balance (topping up) is not the same as renewing (what you call extending).
For me this is the simplest way of doing things as I don't have to bother with figuring out which is the best day to renew on.
Provided there is sufficient funds in your account the plan will renew automatically on the 29th day.
I also make a habit of keeping a small amount of Euros extra in my account as sometimes I need to do something that isn't covered by the plan. For example, accessing your voicemail often isn't part of the plan and you will be charged a small sum if and when you do it.
I like setting up my account so that it makes my life simple.
If I know exactly how many 28 day periods I will be in Spain (and needing a phone) then sometimes when I buy my SIM card from a Vodafone store then I pay the full amount for all periods plus €5 up front.
This way I don't even need to to deposit any further amount into my account or try to navigate the Vodafone website which seems to always have at least one page in Spanish even when I chose the English language options.
The other helpful thing if you are transiting via the UK, which I did this year, is that I found that I had just enough credit in my Vodafone Spain account that I was able to access data in the UK on the first day. This allowed me to use Google Maps to get to my AirBnB.
Data didn't work the next day though.