"It is common knowledge that search results in a jobs database returns the most current jobs first, so posting late on Friday with everyone else may not be the best day/time to update/enter your jobs. By the time those ads run on Sunday and a job hunter gets to his/her computer your Friday job posting could be pages deep into the results. It may be more advantageous to spend a few minutes over the weekend and update your jobs that Sunday morning to get prime first page positioning."

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However, most recruiters still post a lot of jobs at the end of the day Friday (or at the end of the day, period). It's best to sign up for daily job notifications, and to respond to job postings every single day, Saturday and Sunday included. I noticed that there are a lot of rather nocturnal recruiters around here, so if you're up late at night, that's also not a bad time to respond to job postings either.