Recently matched into DO ortho. I rotated from June through October. Each month had more and more students. Your proficiency of the material improves over each month, so by the end of all your rotations you should be performing your best in the later months. Would recommend putting the places you want to go to the most, as your later rotations (see above). Asking for LOR is usually not feasible, as typically you are not with specific attendings for enough time for them to accurately assess your skills (in my opinion). Would recommend having LORs already written by attendings at your home program prior to doing audition rotations.
Most DO programs interview Oct and Nov, so don't rotate after these months. For the MD world, would stagger all above mentioned dates by about 2 months. I think for MD the best months would be from July/Aug - Jan (can't say for certain as I wasn't in the MD match process). Thus, rotating in Jan would probably be the last month you would want to consider. Set up rotations early, and try to go to as many rotations as your school will allow. Gunning for ortho is a full commitment, and if you are on the fence with what field you want to go into, it would be my advice to have a heart-to-heart with yourself prior to diving in head first. Good luck.