I had a similar problem when I went through the Montfort Consecration at this time last year. I had planned to finish the process on the feast day, but a problem arose at work that day and I was unable to get away to go to confession and attend Mass. I spoke with the pastor at the church where I had been doing most of my preparatory activities (he’s an OMI priest, who went through the Consecration himself many years ago, and he helped to guide me through it all.) He said not to worry, just to finish the process as soon as I was able. That turned out to be the next day.
He pointed out that today’s Solemnity is not a Holy Day of Obligation for U.S. Catholics (although many of us think it should be, but that’s another issue!), and since the Consecration is a private devotion, there is no Church law requiring it be done at any particular time. We are not even really obligated to do everything in St. Louis de Montfort’s book, though we should do the best we can in honor of Jesus and Mary (at all times, of course). So long as you feel you have the proper intentions and have faithfully prepared yourself for such a solemn occasion, you’ll be fine, and our Lord and Lady will gladly accept your self-offering.
So, welcome to the “club”, so to speak!! You will find it very rewarding, I am sure.