When Does Medicare Part B Begin? 
If you signed up for Medicare Part A & Part B during your initial enrollment period , then your effective date will be either the first of the month you turn 65 -LRB- this is the case for most individuals -RRB- or 2 or 3 months after -LRB- if you signed up after the month you turned 65 -RRB- . If you decided to wait because you were still working -LRB- e.g. on a group health plan through your employer -RRB- , then you may be eligible for a special enrollment period and set up an effective date for the first of the following month -LRB- you 'll likely need to complete the Part B enrollment at your local social security office -RRB- . If you decided to wait on signing up for Part B and do not have a special enrollment period , then you would be looking at signing up during the general enrollment period -LRB- January 1-March 31 of each year -RRB- . If you 're signing up during this window , then your effective date would be July 1 . 