Yes, you can schedule irregular intervals with crontab by using some special characters. Here are some examples:

1. Running a command every three days:
To run a command every three days, you can use the modulo operator in the day of the month field. For example, the following line will run the command every day that is a multiple of 3:
``` 
0 0 */3 * * /path/to/command
``` 

2. Skipping months:
To skip a month, you can use a comma-separated list of month values in the month field. For example, the following line will run the command every day in January, March, May, July, September, and November:
``` 
0 0 * 1,3,5,7,9,11 * /path/to/command
``` 

3. Running a command once a week one month and twice a week the next month:
To run a command once a week one month and twice a week the next month, you will need to use a combination of the day of the week and day of the month fields. For example, the following line will run the command every Monday in January and every Monday and Wednesday in February:
``` 
0 0 * 1 * [ $(date +\%W) -eq 0 ] && /path/to/command
0 0 * 2 * [ $(date +\%W) -eq 0 -o $(date +\%W) -eq 3 ] && /path/to/command
``` 
Note that the above lines include a shell condition to determine whether to run the command on a particular day. The $(date +\%W) expression returns the current week number, which can be used to determine whether the current day is a Monday (since Mondays always start a new week). The -eq operator is used to compare the week number to the desired day of the week (0 for Monday). The -o operator is used to combine multiple conditions with an OR operator.

If you need to run a more complex scheduling pattern, you may want to consider writing a script that can handle the scheduling logic and then calling that script from crontab. The script can use any programming language that is installed on your system (e.g., Python, Ruby, Perl, etc.).
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