Everyone in the country wants the same three weeks. The fares double and the good rooms go first.
There are only so many seats out of Velana in the school holidays, and every family in the country is looking at the same dates as you are.
What happens in practice is that the cheapest fare class sells out about ten weeks ahead, the next one about six weeks ahead, and what is left in the final fortnight is roughly double what the first people paid. The hotels behave the same way, except that the rooms families want, the ones that take four without a rollaway bed, go earlier still.
So the advice is dull and it works: book in the term before the holiday. Not the month before. If your dates are fixed by school, your only lever is how early you move.
If you cannot commit that early, the next best thing is to be flexible by two days at each end. Leaving on a Tuesday instead of a Friday is frequently the difference between two fare classes, and on a family of four that is real money.
Come in with your dates and we will show you the fare ladder on the screen rather than just quoting you a number.