Venice - Barcelona

Hello.

I have a question. Can anyone tell me how this thing with cheap flights works?
Are some available on special days, or weekends, or are they just randomly offered on
random days? Yesterday I was looking for a flight from Venice to Barcelona, the flight price
was 10 euros per person, but due to financial problems couldn't confirm it yesterday,
and today is gone. I just wanna know from someone experienced can I hope for some
cheap offers in near future, are this like weekend or every other day offers, or just
filling the flight.

Thanks in advance,

Zof