I would like to know how to deal with this situation.

Basically our manager had a clear schedule on who is working and not working over Christmas. I took my vacation accordingly as I was the prime person for the December month-end activities which is a regular part of our job on a monthly basis.

I completed what was necessary on the 23^rd December before leaving on vacation for 3 days and so did my co-worker who was off for the next 4 days.

I was supposed to finish the month-end activities on the 27^th December upon my return, but when I logged in I found my co-worker worked on the 24^th, 25^th, and 26^th of December and finished all the work that was on my plate to finish on the 27^th.

Now, I arrived early in the morning on the 27^th, got online on time, and when I opened up the system I found all this!

I really don't know how to react to this! I feel upset and I am a little bit worried about my co-worker as well!

The reason why I feel upset is because we decided who was going to work on which tasks, I was not late on any of those, and there were absolutely no requirements to work on Christmas and the day after. I planned my work accordingly so that I could stay on the course to finish the month-end activities and now I got up on the 27^th and saw that everything was done over the holidays. I was thinking that if they had said they were going to do this before, I could have planned my vacation accordingly but when the work was distributed they didn't raise this issue at all.

Also, I am worried about my co-worker as they were supposed to rest over the holidays but were still working and maybe they feel some sort of competition or something between the both us.

Could you please advise me how to tackle this issue as this is not the first time my co-worker has done this, working over holidays / weekends and doing my work?