Lucky Friday? You tell me.
I've never seen a study of the daily market moves that reveals that particular days are more likely to post gains -- although there probably is someone who's doing it.
But that's not the idea of a 40l(k). You're supposed to continue investing on a regular basis -- so you'll catch some down days and some up days -- and diversify your portfolio, so that over the long run you'll get that market average return of over 10 percent with dividends reinvested.