  The Dark Energy Survey is a stage III dark energy experiment, performing an
optical imaging survey to measure cosmological equation of state parameters
using four independent methods. The scope and complexity of the survey
introduced complex strategic and tactical scheduling problems that needed to be
addressed. We begin with an overview of the process used to develop DES
strategy and tactics, from the inception of the project, to task forces that
studied and developed strategy changes over the course of the survey, to the
nightly pre-observing meeting in which immediate tactical issues were
addressed. We then summarize the strategic choices made for each sub-survey,
including metrics, scheduling considerations, choice of time domain fields and
their sequences of exposures, and wide survey footprint and pointing layout
choices. We go on to describe the detailed process that determined which
specific exposures were taken at which specific times. We give a chronology of
the strategic and tactical peculiarities of each year of observing, including
the proposal and execution of a sixth year. We give an overview of obstac, the
implementation of the DES scheduler used to simulate and evaluate strategic and
tactical options, and automate exposure scheduling; and describe developments
in obstac for use after DES. Appendices describe further details of data
quality evaluation, tau, and t_eff; airmass calculation; and modeling of the
seeing and sky brightness. The significant corpus of DES data indicates that
the simple scaling relations for seeing as a function of wavelength and airmass
derived from the Kolmogorov turbulence model work adequately for exposure
planning purposes: deviations from these relations are modest in comparison
with short time-scale seeing variations.
