spending fast
n. A period in which a person voluntarily spends as little money as possible.
The organizers of "Not One Damn Dime Day" do not call upon Americans to make sacrifices to end the war in Iraq. In anticipation of a day-long spending fast, it's too easy to stop and fill up the gas tank and buy a quart of milk on the way home today. It's too simple to make a sack lunch tonight for work tomorrow.
—Pam Adams, “'Dime' day a protest, but not a sacrifice,” Peoria Journal Star, January 19, 2005
A spending fast: Whenever we want to save extra money, we have what I call "no spending months." A no-spend month is when money is spent only on essentials. If I want to buy something, I have to ask myself, "Is it essential to our living this month?" Usually, the answer is no.
—Candy B. in Mary Hunt, “Parents, you may appreciate these tips,” St. Paul Pioneer Press, September 04, 2004
2003 (earliest)
When I couldn't buy myself a cup of coffee in the morning (Rule #1: Thou Shalt Not Buy What Thou Can Brew At Home), I went into a tailspin.

Fatal Miscalculation: I'd assumed the spending fast would be easy — because I hadn't given much thought to all the things I spend money on. Or rather, I'd underestimated them. Sure, as I've been careening down the road to financial sanity, I've gotten a grip on most of my big, obvious spending blunders. But who knew there were so many small ones! Well, honey, there ain't nothing like a spending fast to show the dings in your halo.
—MP Dunleavy, “I've given up spending for Lent,” MSN Money, March 05, 2003