Who decides when where and how elections for senators and reps will be held?
For federal offices (President, Vice President, and United States Congress), Election Day occurs only in even-numbered years. Presidential elections are held every four years, in years divisible by four, in which electors for President and Vice President are chosen according to the method determined by each state. Elections to the US House of Representatives and the US Senate are held every two years; all Representatives are elected to serve two-year terms and are up for election every two years, while Senators serve six-year terms, staggered so that one third of Senators are elected in any given general election. General elections in which presidential candidates are not on the ballot are referred to as midterm elections. Terms for those elected begin in January the following year; the President and Vice President are inaugurated ("sworn in") on Inauguration Day, which is usually on January 20.
Who decides when elections for US Senators and US Representatives will be held?
Congress
Election Day (United States)
For federal offices (President, Vice President, and United States Congress), Election Day occurs only in even-numbered years. Presidential elections are held every four years, in years divisible by four, in which electors for President and Vice President are chosen according to the method determined by each state. Elections to the US House of Representatives and the US Senate are held every two years; all Representatives are elected to serve two-year terms and are up for election every two years, while Senators serve six-year terms, staggered so that one third of Senators are elected in any given general election. General elections in which presidential candidates are not on the ballot are referred to as midterm elections. Terms for those elected begin in January the following year; the President and Vice President are inaugurated ("sworn in") on Inauguration Day, which is usually on January 20.
Who decides where elections for US Senators and US Representatives will be held?
states
Election Day (United States)
The Seventeenth Amendment requires that mid-term vacancies in the Senate be filled by special election. Whenever a senator must be appointed or elected, the secretary of the Senate mails one of three forms to the state's governor to inform them of the proper wording to certify the appointment of a new senator. If a special election for one seat happens to coincide with a general election for the state's other seat, each seat is contested separately. A senator elected in a special election takes office as soon as possible after the election and serves until the original six-year term expires (i.e. not for a full term).
Who or what decides how elections for US Senators will be held (i.e. by direct election)?
Seventeenth Amendment
United States Senate
Many state and local government offices are also elected on Election Day as a matter of convenience and cost saving, although a handful of states hold elections for state offices (such as governor) during odd-numbered "off years", or during other even-numbered "midterm years", and may hold special elections for offices that have become vacant. Congress has mandated a uniform date for presidential () and congressional ( and ) elections, though early voting is nonetheless authorized in many states.
Who decides where, when, and how elections for a state government's senators and representatives will be held?
states
Election Day (United States)
Elections to the Senate are held on the first Tuesday after the first Monday in November in even-numbered years, Election Day, and coincide with elections for the House of Representatives. Senators are elected by their state as a whole. The Elections Clause of the United States Constitution grants each state (and Congress, if it so desires to implement a uniform law) the power to legislate a method by which senators are elected. Ballot access rules for independent and minor party candidates also vary from state to state.
According to the US Constitution, who can legislate a method by which senators are elected?
each state (and Congress
United States Senate
Election Day (United States)
https://en.wikipedia.org/wiki/Election%20Day%20%28United%20States%29
Polling place
https://en.wikipedia.org/wiki/Polling%20place
United States Senate
https://en.wikipedia.org/wiki/United%20States%20Senate
From 1789 to 1913, senators were appointed by legislatures of the states they represented. They are now elected by popular vote following the ratification of the Seventeenth Amendment in 1913.
United States Senate
The United States Senate is the upper chamber of the United States Congress, with the House of Representatives being the lower chamber. 
United States Senate
The Senate is composed of senators, each of whom represents a single state in its entirety. Each state is equally represented by two senators who serve staggered terms of six years.
United States Senate
The Elections Clause of the United States Constitution grants each state (and Congress, if it so desires to implement a uniform law) the power to legislate a method by which senators are elected.
United States Senate
The United States Senate is the upper chamber of the United States Congress, and is composed of senators, each of whom represents a single state in its entirety and each state is equally represented by two senators who serve staggered terms of six years. Congress and the states decides when elections for US Senators and US Representatives will be held, while states decide where and how they will be held. From 1789 to 1913, senators were appointed by legislatures of the states they represented, but they are now elected by popular vote following the ratification of the Seventeenth Amendment in 1913. The Elections Clause of the United States Constitution grants each state (and Congress, if it so desires to implement a uniform law) the power to legislate a method by which senators are elected.