There's no single 'best day' to book
It's a widely repeated claim that tickets are always cheaper on a particular day of the week (say, Tuesday afternoon). In reality, airlines use algorithmic, demand-based pricing that varies by route and by season — there's no single universal rule that holds for every flight.
What is worth keeping in mind
One pattern that does generally hold is that weekend (Friday–Sunday) departures and arrivals are often pricier than midweek days, due to higher demand. Booking at the very last minute also tends to be more expensive, since by then only the less favourable fare classes are usually left.
So instead of relying on a specific calendar day, it's better to directly compare actual, current prices across the dates you're considering — which is exactly what fridayjet's flexible date search lets you do, based on live prices.