Yes it's time for that extravaganza known as a "fast" day. A fast it's offically termed despite the very gentle discipline the Church imposes. But I've personally never minded the gentle touch.
The trick is to make it significant enough to get our attention, that is, to focus on what (Who!) really matters. And yet not to make it so significant that it affects our work due to fatigue, low blood sugar, bad attitude, fainting spells, emergency room visits....
The Church prescribes two smaller meals in addition to a "normal" meal. I've heard these mysterious non-normal events as being everything from a "snack" to "half-meal" to "one-brick-shy-of-a-full-meal".
A further complicating factor is energy expenditure. If I shovel snow this morning, as I did, do I get more calories in my snack-half/meal-one-brick-shy meal?
And yet another factor is ye olde "compared to what". One man's half-meal is another's full meal.
Generally, I find if I merely cut my normal meal in half that really doesn't bite (pardon the pun) into my appetite enough to trigger a proper ascetic response.* I find it needs to be approximately 41-42% of a normal meal. I look forward to further studies related to this issue.
And, as always, your mileage may vary, past performance doesn't guarrantee future results, do not remove under penalty of law, etc...
* - proof positive is that I have the energy to write this post.