Load-in and strike are two of the most important times for a tech person, as well as the most hectic.
Load-in is generally the Sunday before the show opens. All the cast and staff are required to be here bright and early. Everyone works to get as much work done as possible, building and moving the set, painting, sewing, making a props table, moving seats into place, getting costumes, etc. The trickiest part, of course, is getting everything done (or as much as possible done) in one day, with the large number of people that will be working in the space, including the actors, director, and producers.
Strike is the final night of the last performance, when you and everyone will tear down the set, return all your rented materials, and take care of business in general. This can actually be a lot of fun, if you like to take things apart and play with power drills.
Some shows (mainly in OFA spaces, such as the NCT) will strike on Sunday following a matinee performance, followed by the next show rigging electrics Monday - Friday, and a set load in Saturday. Sunday strikes are both a blessing and a curse - on the one hand, you don't have to give up your Saturday night, and people generally are more alert and act safer. On the other, people tend to work fast on evening strikes and complete the job quicker (but perhaps at the expense of safety).