Every bank and credit union in the US has a Deposit Agreement and Disclosures document, Bank of America is no different.
Our general policy is to make funds from your cash and check deposits
available to you no later than the first business day after the day
of your deposit. However, in some cases we place a hold on funds that
you deposit by check. A hold results in a delay in the availability of
that sounds great but ...
For determining the availability of your deposits, every day is a
business day, except Saturdays, Sundays, and federal holidays. If
you make a deposit on a business day that we are open at one of our
financial centers before 2:00 p.m. local time, or at one of our ATMs
before 5:00 p.m. local time in the state where we maintain your
account, we consider that day to be the day of your deposit. However,
if you make a deposit after such times, or on a day when we are not
open or that is not a business day, we consider that the deposit was
made on the next business day we are open. Some locations have
different cutoff times.
so if you deposit a check on Friday afternoon, the funds are generally available on Tuesday.
but not always...
In some cases, we will not make all of the funds that you deposit by
check available to you by the first business day after the day of
your deposit. Depending on the type of check that you deposit, funds
may not be available until the second business day after the day of
your deposit. The first $200 of your deposits, however, may be
available no later than the first business day after the day of your
deposit. If we are not going to make all of the funds from your
deposit available by the first business day after the day of your
deposit, we generally notify you at the time you make your deposit. We
also tell you when the funds will be available.
Ok what happens when the funds are available...
In many cases, we make funds from your deposited checks available to
you sooner than we are able to collect the checks. This means that,
from time to time, a deposited check may be returned unpaid after we
made the funds available to you. Please keep in mind that even though
we make funds from a deposited check available to you and you withdraw
the funds, you are still responsible for problems with the deposit.
If a check you deposited is returned to us unpaid for any reason, you
will have to repay us and we may charge your account for the amount of
the check, even if doing so overdraws your account.
Fidelity has a similar document:
Each check deposited is promptly credited to your account. However,
the money may not be available until up to six business days later,
and we may decline to honor any debit that is applied against the
money before the deposited check has cleared.
If a deposited check does not clear, the deposit will be removed from
your account, and you are responsible for returning any interest you
received on it.
I would think that the longer holding period for Fidelity is due to the fact that they want to wait long enough to make sure that the number of times they have to undo investments due to the funds not clearing is nearly zero.