The Talmud in Gittin speaks of people who may lose their wealth because of bad behavior related to Shabbat observance. One of the examples refers to one who examines his properties on Shabbat. He may examine his field to see what needs to be done after Shabbat such as weeding or irrigating. He is already planning for after Shabbat on Shabbat. The other example refers to one who cheapens Shabbat by having a large meal right before Shabbat begins. He is not able to have a proper meal because he has no appetite. We are commanded to only eat a סעודת ארעי, a temporary meal or snack on Friday. The rule is that one should stop eating three hours before candle lighting in order to show proper respect for Shabbat. Just as we are to observe the 39 מלאכות, we are to also observe that which Isaiah spoke of regarding the spirit and anticipation of the holiest day of the week.