Tuesdays are always a good day to check pricing and I suggest doing it each week. While there are exceptions to every rule, the majority of rate changes seem to be available on Tuesdays more than any other day of the week. With airline tickets, it's Tuesday nights.
We have a client who's doing a 14-night cruisetour on Holland America is August. In December, about two months after she booked, I found a drop in price of $120 per person. Last week, I found another price decrease of $500 per person. And today I got her another decrease of $430 per person. That's $1050 per person in the last four months. Needless to say, she's ecstatic about saving $2100 from what she had budgeted for the cruisetour. But all three changes occurred on Tuesdays.