Presbyterian churches practice worship the Triune God, Father, Son, and Holy Spirit. This occurs every Sunday (Lords Day). Other services often occur at other times of the week as well as meetings for prayer and Bible Study or simply mid week chapel with communion being served. Most mainline Presbyterians utilize hymns and with some additions to more modern worship songs being introduced or part of a blended service. Most modern day Presbyterians do not strictly sing Psalms as some once did. John Calvin in Geneva used biblical psalms almost exclusively in the Genevan Psalter, though it contained some gospel canticles and catechetical songs. This psalter was to become a prototype for Reformed worship, but Calvin did not have any objection to the use of original hymns in other churches, and he did not appeal to scripture in his preface to the psalter justifying his preference for the Psalms.:42, 45