The six days between the festival of Sukkot and Shemini Atzeret (five in the Diaspora) and the five days between the first day of Pesach and the last day (four in the Diaspora) are referred to as Chol Hamoed. On Chol Hamoed strenuous work is prohibbited, and the Sages command that Jews must have fun and relax. On Chol Hamoed many Orthodox families go to amusement parks, the circus, the zoo, and engage in other recreational activities with friends to celebrate Passover or Sukkot.
The davar ha'aveid permission to work can also be applicable to income. One is allowed to work to earn money that is needed to forestall a davar ha'aveid, a loss. For example, if someone were to take a week of unpaid leave, and thus not have enough money to pay the rent or mortgage for that month, then he/she is permitted to work. Also, if not working would jeopardize one's job, or realistic chances of promotion, a person may work. Even in places where laws permit taking unpaid leave, in many jobs, taking leave will retard a person's chances of advancement in the company, or even jeopardize one's job altogether, and therefore one is permitted to work..
One is forbidden to plan work for Chol Hamoed, however, even if it will then be a davar ha'aveid, a loss. For example, one is not allowed to put off bringing in the crops in the field until the festival because one will then have more time. If one left the crops in the field, it is forbidden to bring them in during Chol Hamoed, even if this will result in the loss of all the crops. A similar situation is for students who leave their assignments until Chol Hamoed, claiming that then they will have free time. In such a case, it is forbidden to write the assignments during the festival, even if it will result in a loss.
If one cannot plan to leave work until Chol Hamoed when there is more time, what about a salaried worker who has two weeks vacation a year? Can they take a vacation in the summer, when they know that by doing so they will be forced to work during Chol Hamoed? There is much debate between rabbis; however, Rav Moshe Feinstein holds that the essence of "planning" work for the festival involves an active design to put work off until the holiday because a person will then have more time. If the intention is not to deliberately leave work until Chol Hamoed, but rather to simply take a vacation, one is permitted to use their vacation days for the trip. Despite the debate among the Rabbis, many hold that when one wants to save their vacation leave for a good reason, such as travel to Israel, or to take a vacation with the family together to build a shalom bayit, a peaceful home, one is permitted. Rabbis agree that if one needs to take a holiday for health reasons and has to use vacation days, one can work on Chol Hamoed, as otherwise this will result in the davar ha'aved, loss, of one's health.
Sources: "Chol HaMoed-The Intermediate Days." Chabad; Rabbi Kwass, Eliezer, Rabbi Zeff, Joel. "Working on Chol Hamoed: Sources, Status, Nature, and Rules." Darche Noam; >Rabbi Sperling, David. "Working During Chol Hamoed." Nishmat