In general, the market goes up more often than down, essentially restating the historical trend upwards over the available history. Taking that assumption, and extending it to each and every market day, you should have more "up" days than "down" days. So while you could manage to string together a bunch of instances where waiting a few days to invest may happen to coincide with the down days we see every month, the odds would seem to be against you. The counterpoint to this hypothesis is that it assumes that ups and downs are distributed relatively evenly and in similar degree. It would take an enormously larger amount of time to do, but one could look back in time, and see how ups and downs are distributed, and what the average degree of change is, etc. It's possible that while the market on average shows gains over time, these gains are erratically distributed to a high enough degree that your chance of "missing" the large gains goes down relative to what the average would presume. However, as mentioned above, the effects would be miniscule over time, both positive and negative, with regards to weekly/monthly distributions from paychecks and other time-dependent income. So my conclusion is that it's just not worth it to fret over these. For large distributions and windfalls, it might actually be worth looking at, but in those cases, other forum threads have often pointed to academic research that shows on average lump sum investing ends up being superior in hindsight a large majority of the time.