  Since 2004, we have been engaged in a long-term observing program to monitor
young stellar objects in the Orion Nebula Cluster. We have collected about two
thousands frames in V, R, and I broad-band filters on more than two hundred
nights distributed over five consecutive observing seasons. The high-quality
and time-extended photometric data give us an opportunity to address various
phenomena associated with young stars. The prime motivations of this project
are i) to explore various manifestations of stellar magnetic activity in very
young low-mass stars; ii) to search for new pre-main sequence eclipsing
binaries; and iii) to look for any EXor and FUor like transient activities
associated with YSOs. Since this is the first paper on this program, we give a
detailed description of the science drivers, the observation and the data
reduction strategies as well. In addition to these, we also present a large
number of new periodic variables detected from our first five years of
time-series photometric data. Our study reveals that about 72% of CTTS in our
FoV are periodic, whereas, the percentage of periodic WTTS is just 32%. This
indicates that inhomogeneities patterns on the surface of CTTS of the ONC stars
are much more stable than on WTTS. From our multi-year monitoring campaign we
found that the photometric surveys based on single-season are incapable of
identifying all periodic variables. And any study on evolution of angular
momentum based on single-season surveys must be carried out with caution.
