Historical context helpful to an understanding of the parable of the laborers in the vineyard includes the following: 1.) In the history of special relation 'vineyard' is often employed as a metaphor illustrating aspects of God's communion with His covenant people. 2.) A work day consisted of the time from sunrise to sunset. Divide this time by 12, and each of these segments constituted 1 hour. 3.) A denarius a day was an average wage. 4.) The marketplace refers to a center of commerce where labor was secured, employment was sought, contracts were negotiated, goods and services were exchanged etc..
The literary/theological context is also important to recognize. Note that this parable assumes the same basic shape/structure as the preceding events. 1st) Rejection of rich ruler. 2nd) Commentary on salvation.
3rd) Promise to disciples. This pattern corresponds with: 1st) Master of the house/vineyard. 2nd) Laborers hired. 3rd) Payment of wages. The theological emphasis is God's total sovereignty over: property, election, and rewards (wages).