There has been much conversation about the possibility of President Trump filling Cabinet positions while the senate is on recess. On January 8, 2024, the Congressional Research Service put out its updated paper on recess appointments. Below is the paper’s summary statement.

Summary
A vacant presidentially appointed, Senate-confirmed position (advice and consent position) can
be filled temporarily under one of several authorities that do not require going through the Senate
confirmation process. Under specific circumstances, many executive branch vacancies can be
filled temporarily under the Federal Vacancies Reform Act of 1998 or by recess appointment. In
some cases, the authority to fill a vacancy in a particular position on a temporary basis is
specifically provided in statute. Generally, designation or appointment under one of these
methods confers upon the official the legal authority to carry out the duties of the office.
Alternatively, an individual may be hired by the agency as a consultant. A consultant does not
carry the legal authority of the office, and may act only in an advisory capacity.
In many instances, the functions of a vacant advice and consent office have been carried out
indefinitely by another official, usually the first assistant, under the terms of an administrative
delegation order of the agency head. In such instances, the official has carried out these functions
without formally assuming the vacant office on a temporary or permanent basis.

The paper, only ten (10) pages long, is available online for viewing or download at https://crsreports.congress.gov/product/pdf/RS/RS21412/24. It deals with various vacancies, but the section covering recess appointments begins on page 6.

According to Arnold&Porter, recess appointments have not been in use for quite some time. “Since the Supreme Court’s 2014 Noel Canning v. NLRB decision, the Senate’s strategic use of pro forma sessions have blocked Presidents Barack Obama, Donald Trump, and Joe Biden from the conditions needed to make recess appointments.”

Pro forma sessions are meetings of the House or Senate during which no legislative business is conducted and no roll call votes are held. They are held to fulfill the requirement in the U.S. Constitution which prohibits Congress from adjourning for longer than three days without the consent of the other chamber. In practice, this means that when the House or Senate finishes legislative work on Thursday and has no plans to return for legislative work until Tuesday of the following week, it will need to hold a “pro forma session” on Friday or Monday to fulfill the constitutional requirement.
https://www.c-span.org/classroom/document/?2276