Vueling is having another generous sale: €10 one way tickets within Spain.
You need to book during 1st and 2nd of August, for flights from 18th of October 2017 till 20th of March 2018.
Although it says in the advert "fly anywhere in Spain", there are only selected routes you can book. Barcelona has the most routes, but excludes flights to Madrid.

Next best connected city during this sale is Bilbao:

There are few more cities with less connections on sale: Ibiza, Mallorca, etc. From Madrid you can fly for 10 euros only to Ibiza.

The availability overall is not the best: with only a couple of €10 tickets per month, mostly mid-week. One of the reasons is that similar €10 sale was held only a few weeks ago. In any case, there are good chances you can bag a great deal, if your schedule is flexible enough.
Another opportunity is to bag yourself a Vueling Premium – which gives 1 year priority check-in, priority boarding, Fast Track in some airports including London Gatwick (!) and other benefits. In order to get it, you need to have flown or booked 40 one way flights within any calendar year period. Vueling Premium is valid for 1 year. You can read more about Vueling Premium here: http://partners.vuelingnews.com/2014/06/05/introducing-vueling-premium/?lang=en

You can book the flights across Spain with Vueling till the end of the day tomorrow from this offer page: http://www.vueling.com/es/promo-domestic?utm_source=newsletter&utm_medium=email&CRM=PES_EN_201708_Domestic_Routes_20170801&spMailingID=29981355&spUserID=ODM1MTU2NTU1MDIS1&spJobID=1100031406&spReportId=MTEwMDAzMTQwNgS2

Don't forget, that you can earn €2 por transaction on Vueling with cashback website Beruby.com